On Assignment Inc. (NASD: ASGN), a Calabasas, Calif.-based healthcare staffing provider, is acquiring HealthCare Partners Inc., a locum tenens and physician staffing firm.

On Assignment will pay $15.0 million with a two-year earn-out of up to $3.7 million. HealthCare Partners will become part of VISTA Staffing Solutions, On Assignment’s locum tenens and permanent physician search services division.

“HCP is one of the leading locum tenens companies in the country and is a perfect complement to our existing business, both from a geographic and staffing specialty perspective,” said Peter Dameris, president and CEO of On Assignment.

Atlanta-based HealthCare Partners ranked No. 12 on Staffing Industry Analysts’ 2011 list of fastest-growing U.S. staffing firms, with $20.3 million in 2010 revenue. On Assignment, which ranked No. 9 on Staffing Industry Analysts’ 2010 list of largest U.S. healthcare firms, had 2010 revenue of $438.1 million.

Founded by Ruddy Polhill and Rhonda Polhill, HCP has 41 employees and makes physician placements in 14 specialties across 24 states. The Polhills will remain with On Assignment and continue to lead the growth of HCP.

“On Assignment, and its physician staffing division, VISTA, is widely regarded as one of the leading professional staffing companies in the nation,” said Ruddy Polhill, CEO of HCP. “Rhonda and I have known the senior management team at VISTA for many years, and we are excited to become part of On Assignment.”