Herpes simplex virus can be isolated from active epithelial lesions of herpetic keratitis in 60% to 70% of patients, if a single specimen is inoculated into cultures of an established cell line. However, if the patient has received idoxuridine during the 48 hours preceding the specimen, the chance for isolating virus is drastically curtailed. Only 8.6% of such individuals yielded a virus isolate in spite of the presence of active epithelial lesions. Virus isolation can therefore not be employed as a method to confirm the herpetic etiology of epithelial keratitis which continues to exhibit activity during the administration of idoxuridine.