Human Genome Sciences Inc
With 198.5 million shares in issue, the bid would value Human Genome at around $2.6 billion. The stock closed on Wednesday at $7.16.
Human Genome said it did not believe the offer reflected the value inherent in the company. It has hired Goldman Sachs and Credit Suisse to assist with exploration of strategic alternatives.
A spokeswoman for GSK, Britain's biggest drugmaker, said the company would issue a statement shortly.
(Reporting by Ben Hirschler)