The aqueous penetration of orally administered Ciprofloxacin (CPX) was studied in twenty-two human volunteers undergoing cataract surgery. CPX in a dose of 250 mg, was given p.o. at 12 hourly intervals in 3 groups such that a maximum dose of 0.5 grams was delivered in one group, 1.0 grams in another and 1.5 grams in the last group. A fourth group consisted of 6 individuals who were controls and received lactose tablets. Twelve hours after the last dosing aqueous samples were collected and assayed by the Grove-Randall technique. The aqueous penetration of Ciprofloxacin achieved was 1.51±0.15 mgL^–1 in group II and 2.49±0.52 mgL^–1 in group III. In group I, the aqueous concentration was experimentally indeterminate and the control samples in group IV did not show any antibacterial activity. Divided oral doses of 1.5 grams of CPX were found to produce an aqueous titre up to 2.5 times the MIC ofPs. aeruginosa.