Bristol-Myers Squibb has agreed to buy ZymoGenetics for $9.75 per share or about in cash. The transaction, with an aggregate purchase price of approximately $885 million.

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ZymoGenetics, one of Seattle’s earliest and most visible biotechnology companies, has agreed to be acquired by Bristol-Myers Squibb for $9.75 per share, or about $885 million.

It’s unclear how many of ZymoGenetics’ 320 employees will lose their jobs, although spokeswoman Susan Specht acknowledged that “it is typical in a transaction like this that you would have the redundant administrative functions laid off.”

She said Bristol-Myers “has told us it doesn’t have any near-term plans to close the Seattle site until at least through 2011.”

The sale marks the latest acquisition of a major Seattle-area biotech by a pharmaceutical giant, including Amgen’s purchase of Immunex in 2002 and Eli Lilly’s acquisition of Icos in 2007.

Both companies’ boards have approved the deal and ZymoGenetics will recommend shareholders sell their stock to Bristol-Myers in a tender offer slated to begin Thursday.

The price represents an 84 percent premium to ZymoGenetics’ closing price Tuesday of $5.30, before the deal was announced. The company’s stock hasn’t traded above $7 for more than two years.

New York-based Bristol-Myers was evidently most attracted by one product in the ZymoGenetics development pipeline, a possible treatment for hepatitis C infections called pegylated-interferon lambda.

The global pharmaceutical company already has made $200 million in progress payments to ZymoGenetics under a collaboration agreement on the drug inked in early 2009. The drug, known as PEG-interferon, is in Phase IIb clinical trials.

The acquisition, which requires federal antitrust clearance, would be the second time ZymoGenetics has been folded into a larger pharmaceutical company.

Founded in 1981, it was a public company until being acquired by Novo Nordisk in 1988. The Danish company later spun it off and ZymoGenetics went public again in 2002 at $12 a share.

In its latest incarnation as an independent company, ZymoGenetics struggled to develop a successful new product. Its only marketed product, Recothrom, a drug for controlling bleeding after surgery, has met lukewarm reception and reached quarterly sales of just $11.9 million last quarter.

“The acquisition of ZymoGenetics brings us full ownership of a promising investigational biologic that strengthens our very diversified hepatitis C portfolio,” said Bristol-Myers CEO Lamberto Andreotti. “In addition, ZymoGenetics brings proven capabilities with therapeutic proteins and revenue from a marketed specialty surgical biologic.”

Douglas Williams, the former ZymoGenetics research chief who became CEO in 2009, said combining with Bristol-Myers “will maximize the potential for our products and product candidates to make a meaningful difference for patients in need.”

Spokeswoman Specht said ZymoGenetics employees “are very surprised, and hopeful they will be retained.”

They greeted the news with “some sadness here, some hope — mixed feelings,” she said.