55 patients with dendritic ulcer have been treated in a double-blind randomised manner by minimal wiping debridement of their ulcers followed by daily application of either partially purified human interferon (P.I.N.F.) or placebo. There were 6 recurrences of epithelial herpetic disease in 30 patients who received P.I.N.F. and 13 in 25 who received placebo. This provides the long-awaited data proving the antiviral effect of exogenous interferon in treatment of naturally occurring viral disease in man, and furnishes a sighting shot on the lower edge of the target of effective therapy.