A model of bacterial keratitis in rats was developed to quantify the effect of antibiotics and corticosteroid on the infective process. Corneas were inoculated with Staphylococcus aureus, Pseudomonas aeruginosa, or Streptococcus pneumoniae. The natural history of infection with these organisms was determined. Groups of animals received topical antibiotics and prednisolone acetate. The effect of treatment on the number of leukocytes and viable bacteria in the corneas was determined. Prednisolone did not influence the effect of the antibiotics; however, steroid treatment alone increased the pseudomonad count as much as 20-fold above the count in untreated eyes. In general, both the antibiotic and steroid treatments were more successful when begun eight hours after infection than when begun at 24 hours. a 1% gentamicin sulfate preparation proved effective against each of the infections, including a pneumococcal strain considered resistant on the basis of in vitro tests.