GSK has said it opposes bribery and was cooperating with the investigation.
On Wednesday, the Chinese drug regulator launched a crackdown on misconduct in its pharmaceutical market, though it gave no indication it was linked to the GlaxoSmithKline probe.
The State Food and Drug Administration said the campaign is aimed at stamping out unauthorized drug production, improper online drug retailing and sales of fake traditional Chinese medicines.
The new Chinese leadership that took power in November has promised to improve China's health system and rein in surging costs of medicine and medical care that are fueling public frustration.
China has suffered repeated scandals over fake or shoddy medications, some of which caused deaths and injuries. Regulators have launched repeated crackdowns on false advertising and other violations, but with limited success.
Also Wednesday, a Commerce Ministry spokesman warned that Chinese and foreign drug manufacturers would face "legal sanctions" for misconduct.
Meanwhile, the Cabinet's planning agency is investigating production costs at 60 Chinese and foreign pharmaceutical manufacturers, according to state media, possibly a prelude to revising state-imposed price caps on key medications.