Combinations of neomycin sulfate and polymyxin B sulfate are commonly used in ophthalmic ointments for the treatment or the prevention of bacterial keratoconjunctivitis. In this study we evaluated the effectiveness of various ointments containing these two antibiotics, alone and in combination, in preventing Staphylococcus aureus keratitis in rabbits. Rabbit eyes were infected by intracorneal inoculation, treated topically with ointment and graded by gross observation 24 hours after inoculation. Treatment with ointments containing neomycin alone offered significant protection against these corneal infections. The polymyxin B ointments, as well as the vehicle controls, were ineffective in preventing S aureus infections in the rabbit eyes. However, by far, the most effective ointment formulations tested were the combination ointments and specifically those containing 1.75-3.50 mg neomycin and 3,000-6,000 units polymycin B per gram of ointment.