"The rights plan is intended to protect RCM and its stockholders from efforts to obtain control of RCM that are inconsistent with the best interests of RCM," the company wrote.

The stockholders plan would come into effect if a person or group acquires 15% or more of RCM's common stock without board approval or if an entity already owning 15% or more in stock acquires further shares. In those events, the plan would allow all shareholders -- other than the party acquiring the unauthorized stake -- to buy RCM stock at a 50% discount.

RCM last week publicly rejected a bid to be acquired by CDI, saying it was not in the best interests of the company. CDI had offered $5.20 per share in cash in a deal worth approximately $68 million based on 12,999,178 shares.

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Pennsauken NJ-based RCM provides engineering, information technology and commercial staffing. It ranked No. 70 on Staffing Industry Analysts' 2009 list of largest U.S. staffing firms.

CDI, based in Philadelphia, provides engineering and IT services as well as staffing. It ranked No. 27 of last year's list of largest U.S. staffing firms.