A comparative bioavailability study of a single (500 mg) oral dose between ampicillin plus probenecid (Ampilong DS(R), American Remedies Ltd., India) and ampicillin alone (Roscillin(R), Ranbaxy Lab., India) was carried out in 8 healthy male volunteers. The study was designed in a cross-over fashion. Blood samples were collected at 0, 0.5, 1, 1.5, 2, 3, 4, 6, 9, 12 and 24 h after drug administration and ampicillin levels were measured using microbiological technique. Results indicated a better rate and extent of bioavailability of Ampilong DS(R) as compared to Roscillin(R). The Cmax, Tmax, AUC(0-$α$) and elimination t( 1/2 ) of ampicillin were significantly higher with oral Ampilong DS(R). The higher serum ampicillin levels with Ampilong DS(R) is due to the presence of probenecid. Hence, Ampilong DS(R) may have wider spectrum of activity as compared to Roscillin(R).