The efficacy of trifluorothymidine was compared with idoxuridine on a blind basis by noting the effect of topical administration of the drugs on the severity of vaccinial keratitis induced in rabbits. Trifluorothymidine was found to be substantially more effective, and was well tolerated. In addition, positive viral cultures after the treatment period were substantially reduced in the trifluorothymidine groups when compared with the idoxuridine or control groups.