Acycloguanosine (9-[2-hydroxyethoxymethyl]guanine) (Wellcome 248U) has a highly specific and potent antiviral action against herpes-simplex virus (H.S.V.) types I and II in cell-culture systems and in animal models of H.S.V. infection in the brain, skin, and eye. Its efficacy in man was tested in twenty-four patients with dendritic corneal epithelial ulcers treated by minimal wiping debridement. Patients were randomly allocated to treatment with 3% acycloguanosine eye ointment or placebo. There were seven recurrences or recrudescences of typical corneal epithelial herpetic lesions within 1 week of debridement in the twelve patients who received placebo, and no recurrences in the twelve who received acycloguanosine. Four further patients with dendritic ulcers have been electively treated with topical acycloguanosine alone. Their ulcers all healed briskly. No adverse effects were seen with the acycloguanosine therapy. These results establish that acycloguanosine is a clinically effective antiviral drug against H.S.V. infection in man.