Analysts speculated that those two companies, which have similar specialties to Medarex, could be the next to be acquired.
The $2.4 billion deal for antibody technology specialist Medarex, announced late on Wednesday, marked the latest in a string of windfalls for biotech investors this year.
The industry has also seen a wave of out-sized share price gains in 2009 on the back of positive clinical data and deals.
Regeneron was up 9 percent at $19.60 in afternoon trading, while Seattle Genetics was 6.9 percent higher at $9.97.
Like Medarex, both of those companies specialize in monoclonal antibodies. Shares of ImmunoGen (Nasdaq:IMGN) another monoclonal antibody specialist, were up 3.9 percent.
"In the wake of the Medarex acquisition, I see Regeneron as one of the last remaining companies with a true (monoclonal antibody) platform that is still stand alone," ThinkEquity analyst Jason Kolbert said.
Just as Medarex had struck partnership deals, Kolbert said Regeneron has ties to large companies such as Bayer and Sanofi-Aventis.
"They have a great balance sheet, great partnerships with pharma giants," Kolbert said. "It's got all of the same ingredients that a Medarex has."
Regeneron and Seattle Genetics are the most likely next candidates to be acquired, said Mark Monane, an analyst at Needham & Co.
Monane said other characteristics that define likely biotech takeout targets included companies with hematology and oncology products, medicines in late-stage development and existing partnerships with other companies.
Monane said that other companies that could be put in play, although somewhat less likely, include Alexion Pharmaceuticals (Nasdaq:ALXN), Allos Therapeutics (Nasdaq:ALTH), AMAG Pharmaceuticals (Nasdaq:AMAG), ArQule (Nasdaq:ARQL), Celldex Therapeutics (Nasdaq:CLDX), Dendreon (Nasdaq:DNDN) and Poniard Pharmaceuticals (Nasdaq:PARD).
Dendreon shares rose 6.6 percent to $24.57 after a source told Reuters the biotech company hired JPMorgan Chase as an adviser to help it find an international licensing partner for its prostate cancer vaccine Provenge.
Many other large drugmakers are scrambling to buy promising biotechnology companies to fill their late-stage pipelines as they confront generic competition to big-selling products.
Indeed, Bristol-Myers CEO James Cornelius said the company remains financially able to pursue more deals even with the Medarex deal.
Earlier this month, Johnson & Johnson (NYSE:JNJ) completed its $930 million purchase of Cougar Biotechnology Inc, just days after agreeing to pay $1 billion for an 18 percent stake in Irish drugmaker Elan Corp (NYSE:ELN) and to acquire a major stake in Elan's portfolio of experimental treatments for Alzheimer's disease.
Several biotech companies have seen their shares soar this year after surprising investors with positive clinical data or regulatory approvals.
Just this week, Human Genome Sciences (Nasdaq:HGSI) shares jumped more than 200 percent after the company announced positive data for its experimental lupus drug.
Dendreon and MAP Pharmaceuticals (Nasdaq:MAPP) are among the other companies with positive surprises this year.
(Reporting by Lewis Krauskopf and Bill Berkrot; Editing by Ted Kerr and Richard Chang)
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