Introduction Ampicillin is a new broad-spectrum penicillin that should be a useful and safe antimicrobial agent for the treatment of ocular infections. This agent is an acid stable semisynthesized substance and as such is absorbed well after oral administration. It is currently available from Bristol Laboratories for oral use as Polycillin, 250 mg capsules. This study was undertaken to determine the ocular penetration of ampicillin in human subjects after oral administration. Ampicillin levels in aqueous and blood plasma were determined after administration of a single dose of 250 mg, 1,000 mg, or 2,000 mg orally. In addition, the ocular and plasma levels were determined in several patients who received multiple doses of ampicillin. Studies at the relatively low dosage of 250 mg were prompted by the previous observations of its excellent ocular penetration in rabbits1 and the suggestions by Bunn et al2 that a dose of 250 mg