Updated from 3:22 p.m.

Express Scripts ( ESRX) is turning up the heat on Caremark ( CMX).

St. Louis-based Express Scripts said Monday it will wage a proxy fight to replace four directors on the board of Caremark, its Nashville, Tenn.-based mail-order pharmacy rival.

The news comes just hours after Caremark rejected Express Scripts' unsolicited $26 billion cash-and-stock bid in favor of a $21 billion all-stock merger with CVS ( CVS).

"Since we made our offer for Caremark public, Caremark stockholders and the marketplace as a whole have demonstrated their strong support for our offer," Express Scripts said. "We clearly provide Caremark stockholders with superior value to the proposed acquisition of Caremark by CVS. The advantages of an Express Scripts-Caremark combination are compelling."

Indeed, some observers have wondered aloud how Caremark's board can justify accepting the CVS bid when it's worth substantially less than Express Scripts'.

But late Monday, CVS indicated it isn't concerned about what shareholders might think.

"Express Scripts' nomination of four directors to Caremark's board is nothing more than a publicity stunt," it said. "We continue to look forward to the planned first quarter close of our merger with Caremark.

"With the antitrust waiting period for the CVS/Caremark merger having already expired without a second request, and with our proxy statement already on file we are well on our way to completing our transaction, months before the Caremark annual meeting that would have occurred in May."

After trading lower for most of the day, Caremark shares jumped on news of the proxy fight. They closed up 29 cents at $56.64. That's well above the indicated value of the CVS deal, which is just north of $52 a share at recent prices, but below the Express Scripts bid, which is worth $58 and change.

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