To study the possibility of establishing a rapid diagnosis of herpes simplex keratitis, corneas of 10 rabbits, a total of 20 eyes, were inoculated with herpes simplex virus (HSV), type I, strain PH. Epithelial keratitis developed within three days of inoculation in all the animals used. Scraping of infected corneas were smeared and examined, using a modified indirect immunoperoxidase technic. One hundred percent of the smears prepared from these corneas demonstrated positive cells. Negative findings in corneas inoculated with adenovirus 19 suggest the specificity of the reaction. To test the possibility of blockage of staining by the presumed development of circulating endogenous anti-HSV I antibodies, the corneas of eight consecutive patients who presented to the Albany Medical Center Hospital with known recurrent dendritic keratitis also were scraped and stained, using a similar procedure. Positive cells present in each of these scrapings suggest against the blocking of this immunoperoxidase method by the development of circulating anti-HSV antibodies.