• Human buffy-coat interferon and a leukocyte subtype (LeIF-A) obtained by gene cloning in Escherichia coli were equally effective in ameliorating the course of herpetic keratitis in rabbits when applied topically at a daily dose of 0.5 x 106 units per eye for six days commencing one day before infection. When given intramuscularly at the same dose, there was no alteration in the course of the disease. Topical treatments with LeIF-A were equally effective in suppressing epithelial damage seven days after infection, regardless of whether treatment commenced one day before infection or two days after infection. However, treatment from the day before infection also caused a significant decrease in epithelial defect two days after infection.