The majority of ocular infections in the industrialized countries arise after surgical interventions. An appropriate antibacterial prophylaxis is therefore highly desirable in eye surgery. We used high performance liquid chromatography (HPLC) to measure the concentrations reached by imipenem, a modern wide-spectrum beta-lactam antibiotic, in plasma and aqueous humor of 26 patients scheduled for cataract surgery. Even a single 500 mg dose of imipenem achieved therapeutic levels of the molecule for the most common ophthalmic pathogens (1 microgram/ml or above) in the aqueous humor within one to two hours after administration. This drug may therefore be suitable for antibacterial prophylaxis in eye surgery.