The safety and efficacy of a subunit herpes simplex virus (HSV) type 1 vaccine were tested in a small-scale double-blind trial carried out in a group of 42 volunteers suffering from frequent recurrences of herpetic lesions. The patients were paired according to sex, age, type of virus isolated, previous history of the disease and some non-specific immunological markers. One member of each pair received repeated doses of HSV vaccine, the other a placebo. Clinical reactions were mild. Antibody responses following the vaccination were generally low and were almost entirely limited to subjects suffering from HSV-2 lesions. A majority of the patients exhibited improvement of their condition during the postvaccination period. These improvements were, however, nearly equally distributed between the vaccine and placebo groups.