"The Board welcomes involvement by shareholders and is prepared to review any proposals that might result in a Superior Proposal per the merger agreement," Minneapolis-based Ceridian wrote on Wednesday.
Last month, the company agreed agreed to be bought by buyout firm Thomas H. Lee Partners and insurer Fidelity National Financial Inc. for $36 a share. Ceridian said on Wednesday this deal "provided the greatest and most certain value of the available alternatives."
Hedge fund Pershing Square Capital Management, which owns 14.9 percent of Ceridian, however, is disappointed by the proposed takeover price and has hired investment bank Lazard Freres & Co. and law firm Sullivan & Cromwell to find an alternative to the current plan.
"We do not support a sale of the company at this low price," Pershing's founder, William Ackman, wrote to Ceridian shareholders. "It appears to us that the current deal is an ill-suited response to our proxy contest, and is suboptimal for Ceridian stockholders.
Ackman has added to his position in Ceridian, adding roughly 900,000 shares in the last days.
His campaign illustrates how influential activist investors like hedge funds can be in pressuring publicly traded companies to do better for their shareholders.
Ackman is proposing to replace Ceridian's board with an independent slate and has long pressured the company to improve performance by splitting up its two main units -- the human-resources division and the credit-card payment procession division. Ceridian said it plans to hold its shareholder meeting on or before September 21.
Since Ceridian announced plans to sell itself, other private equity buyers and strategic partners have expressed interest in the company, Ackman told shareholders in the letter. Among some of the alternatives to the current deal, Ackman said he would like to see Ceridian sold for a higher price or split apart.
Ackman declined to comment on the matter and would not say who the potentially interested parties may be.
Ceridian's share price traded up 18 cents, or 0.5 percent, to $35.39 on Wednesday.
Ackman has also not put a preferred price tag on the company but gave some hints in a securities filing. In addition to earning its retention fee, Lazard would be paid $1.5 million if the shares are sold at $38.50 or more, the filing said.