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ON
ASSIGNMENT INC
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(Name
of Issuer)
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Common
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(Title
of Class of Securities)
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68215910
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(CUSIP
Number)
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December
31, 2009
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(Date
of Event Which Requires Filing of this
Statement)
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1.
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NAME
OF REPORTING PERSONS
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I.R.S.
IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)
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William
Blair & Company, L.L.C.
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36-2214610
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2.
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CHECK
THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
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(a) o
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(b) o
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3.
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SEC
USE ONLY
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4.
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CITIZENSHIP
OR PLACE OF ORGANIZATION
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222
W Adams
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Chicago,
IL 60606
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NUMBER
OF
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5.
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SOLE
VOTING POWER
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SHARES
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1911929
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BENEFICIALLY
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6.
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SHARED
VOTING POWER
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OWNED
BY
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-0-
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EACH
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7.
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SOLE
DISPOSITIVE POWER
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REPORTING
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1911929
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PERSON
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8.
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SHARED
DISPOSITIVE POWER
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WITH
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-0-
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9.
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AGGREGATE
AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
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1911929
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10.
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CHECK
BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN
SHARES*
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o
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11.
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PERCENT
OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
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5.30%
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12.
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TYPE
OF REPORTING PERSON*
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BD,
IA
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Item
1(a).
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Name
of Issuer:
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Item
1(b).
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Address
of Issuer's Principal Executive
Offices:
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Item
2(a).
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Name
of Person Filing:
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Item
2(b).
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Address
of Principal Business Office, or if None,
Residence:
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Item
2(c).
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Citizenship:
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Item
2(d).
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Title
of Class of Securities:
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Item
2(e).
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CUSIP
Number:
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Item
3.
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If
This Statement is Filed Pursuant to Rule 13d-1(b), or 13d-2(b) or (c),
Check Whether the Person Filing is
a:
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(a)
x
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Broker
or dealer registered under Section 15 of the Exchange
Act.
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(b)
o
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Bank
as defined in Section 3(a)(6) of the Exchange
Act.
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(c)
o
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Insurance
company as defined in Section 3(a)(19) of the Exchange
Act.
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(d) o
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Investment
company registered under Section 8 of the Investment Company
Act.
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(e)
x
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An
investment adviser in accordance with Rule
13d-1(b)(1)(ii)(E);
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(f)
o
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An
employee benefit plan or endowment fund in accordance with Rule
13d-1(b)(1)(ii)(F);
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(g) o
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A
parent holding company or control person in accordance with Rule
13d-1(b)(1)(ii)(G);
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(h)
o
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A
savings association as defined in Section 3(b) of the Federal Deposit
Insurance Act;
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(i)
o
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A
church plan that is excluded from the definition of an investment company
under Section 3(c)(14) of the Investment Company
Act;
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(j)
o
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Group,
in accordance with Rule
13d-1(b)(1)(ii)(J).
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Item
4.
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Ownership.
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(a)
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Amount
beneficially owned:
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(b)
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Percent
of class:
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(c)
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Number
of shares as to which such person
has:
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(I)
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Sole
power to vote or to direct the vote
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(ii)
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Shared
power to vote or to direct the vote
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(iii)
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Sole
power to dispose or to direct the disposition
of
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(iv)
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Shared
power to dispose or to direct the disposition
of
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Item
5.
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Ownership
of Five Percent or Less of a Class.
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Item
6.
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Ownership
of More Than Five Percent on Behalf of Another
Person.
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Item
7.
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Identification
and Classification of the Subsidiary Which Acquired the Security Being
Reported on by the Parent Holding Company or Control
Person.
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Item
8.
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Identification
and Classification of Members of the
Group.
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Item
9.
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Notice
of Dissolution of Group.
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Item
10.
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Certifications.
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February
5, 2010
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(Date)
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/s/
Michelle Seitz
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(Signature)
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Principal
& Manager of Investment Services
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(Name/Title)
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