Pfizer's offer of $90 per share represents a premium of 39 percent to Hospira stock's close on Wednesday. Hospira was trading at $88 before the bell, while Pfizer was up 1.7 percent.
Drugmakers are racing to develop biosimilars, which typically cost 20 percent to 30 percent less than the original, as big-ticket patents on biotech drugs expire and cash-strapped healthcare systems look to cut costs.
Biosimilars are expected to account for about a quarter of the $100 billion in sales stemming from off-patent biological drugs by the end of the decade, according to a study compiled by Thomson Reuters BioWorld.
Hospira is seeking approval from the U.S. Food and Drug Administration to market a copy of Johnson & Johnson's <JNJ.N> blockbuster arthritis treatment Remicade.
The total enterprise value of the deal is about $17 billion, the companies said.
Hospira had $1.75 billion of outstanding long-term debt as of Sept. 30, according to a regulatory filing.
The deal is expected to add 10 cents to 12 cents per share to Pfizer's earnings in the first full-year after the deal closes.
Pfizer's financial advisers are Guggenheim Securities, J.P. Morgan and Lazard, with Ropes & Gray serving as legal adviser and Clifford Chance advising on international regulatory matters.
Morgan Stanley is Hospira's financial adviser, while Skadden, Arps, Slate, Meagher & Flom & Affiliates served as its legal adviser.