Express had launched a hostile takeover bid for Caremark in an effort to wrest Caremark away from its current $24 billion agreement to be acquired by drug-store chain CVS.
"We cannot in good conscience offer more consideration without an opportunity to conduct confirmatory due diligence," Express Scripts CEO George Paz said in a statement.
"If we were able to identify additional value during confirmatory due diligence, including if we determine that there are greater net synergies beyond what we have reflected in our analysis thus far, it could result in an increase to our offer price," Paz said.
Caremark has refused to negotiate with Express Scripts.
CVS last week raised its offer for the third time to $54.12 a share, plus a one-time dividend of $7.50 per share payable after closing of the deal. Express Scripts had bid $61.63 a share, or $26.9 billion, for Caremark.
Although Express Scripts has offered more money, a deal with Caremark would not close until the third quarter of 2007 and would face regulatory review and shareholder approval.
Caremark's deal with CVS could close shortly after shareholders vote later this week. CVS shareholders will vote on the Caremark pact on March 15, while Caremark shareholders are scheduled to vote on March 16.
Caremark also has slammed the Express Scripts bid as being highly conditional and full of financing and regulatory risks.
Caremark and CVS could not be immediately reached for comment.