Moxalactam was administered subconjunctivally in 100-mg doses to rabbits with infected eyes (Staphylococcus aureus endophthalmitis). High concentrations of drug were detected in the sclera, cornea, and choroid; much lower levels were found in the retina, whereas peak concentrations in the vitreous were about 6 microgram/ml. Repeated intramuscular injections of 50 mg/kg every 4 h produced peak serum levels of about 100 microgram/ml. A gradient between the choroid and the retina was again evident, and peak vitreous levels were about 6 microgram/ml after six injections. These data are consistent with the concept of a blood-retina barrier analogous to the blood-brain barrier. Moxalactam appears to penetrate the eye somewhat better than do other beta-lactams; however, the peak levels produced in the vitreous humor in this animal model were below the level required to inhibit most strains of Pseudomonas aeruginosa.