Two private equity firms struck a deal to buy CHG Healthcare Services, the second-largest U.S. healthcare staffing firm, in a transaction that is expected to close by the end of this year. CHG had estimated U.S. healthcare staffing revenue of $597 million in 2011. However, terms of the transaction were not disclosed.

CHG’s existing management will keep a significant stake in the company after the deal.

The firms acquiring CHG include Leonard Green & Partners and Ares Management LLC. Leonard Green is based Los Angeles and has raised more than $15 billion in private equity capital since it was founded in 1989. Ares is also based in Los Angeles and has $54 billion in committed capital under management. Ares recently acquired Insight Global, one of the largest and fastest-growing information technology staffing firms.

Current majority owner of CHG is J.W. Childs Associates, a Boston-based private equity firm.

“We appreciate the positive, successful partnership we’ve enjoyed with J.W. Childs for the last six years, and look forward to continuing our growth with our new partners at Leonard Green and Ares,” said CEO Michael Weinholtz.

Salt Lake City-based CHG was founded in 1979 and is among the nation’s oldest healthcare staffing firms. Its brands include CompHealth, Weatherby Healthcare, RN Network and Foundation Medical Staffing.

CHG is the second-largest U.S. healthcare staffing firm and the largest U.S. provider of locum tenens. Staffing Industry Review magazine ranked the company among the “Best Staffing Firms to Work For” this year.