W e found that intravitreally injected carbenicillin (Geopen@) in a dose of 7.0 mg or lower is nontoxic to all intraocular structures. A 3.0 mg dose produces levels bactericidal for many strains of Pseudomonas aeruginosa up to 24 hours. Experimentally induced Pseudomonas endophthalmitis was successfully treated 10 hours after inoculation by a single intravitreal injection of 5.0 mg of carbenicillin. Eyes treated intravitreally at 24 hours and the eyes of rabbits treated intramuscularly with carbenicillin all progressed to phthisis bulbi. The success of intravitreal treatment is inversely related to the time lapse between inoculation and therapy.