The aqueous humor penetration of the third-generation cephalosporin, ceftizoxime, was determined in 21 patients awaiting cataract extraction, after an intravenous bolus injection of 2 g. The beta-lactam compound achieved a mean peak aqueous humor concentration of 7.8 mg/l two hours after administration with a serum/humor percent ratio ranging between 7 and 16% during the four-hour sampling interval. Therefore, ceftizoxime levels in this extravascular fluid were above the minimum inhibitory concentrations for the majority of sensitive organisms.