Updated from 11:50 a.m. EDT

ImClone Systems ( IMCL) on Wednesday said it had received a $70-a-share bid from a large, unnamed pharmaceutical company, officially rejecting a lower bid from Bristol-Myers Squibb ( BMY - Get Report) in the process.

The new bid comes after activist investor Carl Icahn, ImClone's chairman and an outspoken critic of the $60-a-share Bristol-Myers bid, "has had several conversations with" the CEO of a large, unnamed pharmaceutical company, ImClone said in a statement.

The new bid is a 51% premium to ImClone's closing price the day before the first bid. ImClone shares were up 7% to $68.08 in recent trading Wednesday.

Street watchers have placed German company Merck KgaA, which sells ImClone's Erbitux in Europe, high on their suspect list for the mystery bidder. The pharmaceutical company, like its peers, could be looking to take advantage of the favorable exchange rate -- and it would be a big advantage to own ImClone, says Carol Werther, biotech analyst for Summer Street Research.

Merck KgaA was aggressive about the promising outlook for Erbitux on its last quarterly conference call and at the American Society of Clinical Oncology annual meeting in May, notes Werther.

There are other suspects as well. Senior Columnist Adam Feuerstein said in a post on sister site RealMoney.com that another possibility is Sanofi Aventis ( SNY - Get Report), which has a brand new CEO who could be shaking things up.

Also, Japanese drugmakers have shown an interest in U.S. companies. Takeda took out Millennium Pharma in a $9 billion deal, setting an expensive market for its competitors that may be shopping in the U.S.

ImClone said in a statement on Wednesday that its special committee has decided to let the new bidder conduct due diligence for a two-week period. Icahn said ImClone has not determined, however, whether the new offer is adequate.

Icahn affected the sale of MedImmune to AstraZeneca ( AZN - Get Report) for more than $15 billion, and a per-share price that represented a 53% premium to the closing price of MedImmune's shares the day prior.

Icahn has said in the past that ImClone is worth $80 a share, says Summer Street's Werther.

Not surprisingly, ImClone's board said in August that Bristol's offer of $60-a-share for the 83% of the company that it doesn't already own -- which valued ImClone at roughly $5.2 billion -- "substantially undervalued the company." ImClone did, however, form a committee "to study the matter and to retain advisors to advise it in determining the appropriate course of action."

The board said it had also been discussing separating ImClone's lead product, cancer drug Erbitux, from the its pipeline.