Professional staffing company On Assignment Inc. closed on its $600 million purchase of Apex Systems today and raised its second quarter outlook to reflect the acquisition.

Second quarter earnings should be between 23 and 24 cents a share compared to the previous estimate of 20 to 22 cents a share, the company said. The company also said it expected second quarter revenue to rise to $278 million to $280 million, up from the previously expected revenues of $177 million to $180 million.

Apex Systems was a leading information technology staffing firm based in Richmond, Virginia. Calabasas-based On Assignment, which is in the broader placement business, announced the acquisition earlier this year for $600 million in cash, stock and debt.

Prior to the purchase, Apex was the 6th largest IT staffing firm with revenues of $700 million growing at a compounded annual rate of more than 30 percent. The purchase makes On Assignment the second largest IT staffing firm in the country. The combined companies would have had 2011 pro forma revenues of $1.3 billion.

Shares of On Assignment rose 28 cents today, closing at $16.23, down from a peak of $18.98 on April 27.

Judy Temes