GlaxoSmithKline, the British drug maker secured the US research partner Human Genome Sciences Inc. after raising its bid to $3.0 billion (2.4 billion euros).
GSK said in a statement that it won approval from HGS management after lifting its bid to $14.25 per share in cash from $13 per.
"GlaxoSmithKline Plc and Human Genome Sciences today announced that the companies have entered into a definitive agreement under which GSK will acquire HGS for $14.25 per share in cash.
"The transaction values HGS at approximately $3.6 billion on an equity basis, or approximately $3.0 billion net of cash and debt."
Glaxo and biotechnology company HGS jointly sell lupus drug Benlysta and share financial interests in experimental drugs for diabetes and heart disease.
GSK launched a $2.6-billion hostile bid in May after HGS rejected an informal approach the previous month.
Monday's agreed price represents a premium of 99 percent to the HGS closing share price on April 18, 2012, which was the last day of trading before the informal bid was disclosed.
"We are pleased to have reached a mutually beneficial agreement with HGS on friendly terms and believe the combination of GSK and HGS represents clear financial and strategic logic for both companies and our respective shareholders," GSK chief executive Andrew Witty said in the statement.
"This is a natural next step in our nearly 20-year relationship with HGS."
The deal was agreed on the day that Human Genome Sciences had set as the deadline for the process of finding alternative suitors.
"After a thorough analysis of strategic alternatives, HGS has determined that a combination with GSK is the best course of action for our company and the best way to maximize value for our stockholders," said HGS chief executive H. Thomas Watkins.
"HGS has had a long and productive working relationship with GSK, and together we will be uniquely positioned to achieve the full potential of Benlysta and other products in our pipeline for the benefit of those battling serious disease around the world."
In late afternoon deals, GSK shares gained 0.52 percent to 1,453.50 pence on London's FTSE 100 index of leading companies, which was 0.12 percent lower at 5,659.70 points.
- Dow Jones Newswires contributed to this report -