Shares in British drugmaker Shire jumped as much as 10 percent on Friday, as traders reported talk of bid interest from U.S. rival Pfizer.
But some analysts thought a tie-up was unlikely, saying Shire's focus on treatments for attention deficit hyperactivity disorder (ADHD) and enzyme replacement therapies did not fit with Pfizer's drugs portfolio or its pipeline of new medicines.
Shire declined to comment, while Pfizer could not immediately be reached for comment.
"We don't see why Pfizer would want to go into ADHD and enzyme replacement therapy," said Canaccord Adams analyst Guillaume van Renterghem.
"We think it's very, very unlikely -- it doesn't fit well with Pfizer's pipeline or current portfolio."
Shire's shares were up 7.5 percent at 1,014 pence, off an earlier high of 1,035 pence and valuing Britain's third-biggest drugmaker at about 5.8 billion pounds ($11.8 billion).
More than two million shares changed hands in the first 100 minutes of trade, compared with a daily average of 6.5 million shares in the past 30 days.
Shire has long been tipped as a bid target, because of its strong portfolio of recently-launched drugs, at a time when many bigger pharmaceutical companies are struggling to get new medicines to market.
However, the stock has fallen around a quarter from its highs last year, following slower-than-expected growth of its biggest new drug hope, Vyvanse for ADHD.
Pfizer, the world's biggest drugmaker, is under pressure to come up with a plan to offset looming generic competition to its $12 billion-a-year cholesterol fighter Lipitor.
It said last week that it planned to expand in emerging markets and to speed up the launch of new drugs.
Anglo-Swedish drugmaker AstraZeneca has been tipped as a potential bidder for Shire in the past.