Shares of health insurer Centene (CNC) - Get Centene Corporation Report  were down sharply Monday after rival Humana (HUM) - Get Humana Inc. (HUM) Report said that it would not place a bid for the company.

Humana shares were rising 3.9% in trading.

"Humana Inc.'s long-standing policy is not to comment on rumors or speculation regarding possible M&A activity. However, in light of the significant investor speculation and persistent market rumors regarding the company's intentions with respect to pursuing a combination with Centene Corporation, the Company has chosen to make a one-time, limited exception to its no-comment policy (which continues in effect) and confirm that the company will not make a proposal to combine with Centene as an alternative to Centene's proposed transaction with WellCare Health Plans, Inc. The company does not intend to make any further statement regarding the foregoing," the company said in an SEC filing Monday. 

Centene announced the purchase Wellcare Health Plans (WCG) - Get WellCare Health Plans, Inc. Report for $15 billion in March. The combined company would be the fourth-largest Medicare company in the country with about 22 million members. 

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The tie-up still has to be approved by Wellcare shareholders. Centene shares are down 7.3%.