Tobramycin is an aminoglycoside antibiotic newly marketed for topical ophthalmic use. In a double-masked, multicenter study, tobramycin and gentamicin sulfate, the latter an established topical aminoglycoside, were evaluated in the treatment of patients with bacterial infections of the external eye. Efficacy was determined by resolution of signs and symptoms and the follow-up impression made by the physician. A quantitative, bacterial assay of the conjunctiva and skin-lash margin before and after treatment was used to determine the antimicrobial effectiveness of the two antibiotics. The results of the study, involving 511 patients, indicate that tobramycin is significantly more effective than gentamicin sulfate clinically, and the former exhibits a greater antibacterial efficacy, in the eye, against the combined conjunctival pathogens. On the conjunctiva, Staphylococcus aureus is significantly more susceptible to tobramycin than gentamicin sulfate. Although the solutions are equally safe, tobramycin ointment is associated with a significantly lower frequency of adverse reactions than gentamicin sulfate ointment. This may be due to the preservative used in gentamicin ophthalmic ointment, methyl and propyl paraben.