The preparation and early clinical experience with "antigenoid" vaccine Ac NFU1(S-) MRC is described. A neutralising antibody response against both type 1 and type 2 herpes simplex virus was stimulated in 19 of 21 and 25 of 60 patients who received the high and low dosage of vaccine, respectively: immunoprecipitating antibody against type-common virus antigen "band II", a virus antigen of major importance in neutralization of herpes simplex virus, was detected in 12 of 19 and 2 of consorts who received the high and low dosage vaccine, respectively. To date, after a mean follow-up period of one year, none of the 42 consorts have contracted herpes genitalis: following a mean follow-up period of 6 months, only 1 patient vaccinated after the initial clinical episode has reported a recurrence of herpetic disease. These results compare favourably with the rates of consort transmission and first year recurrences in unvaccinated subjects. Seventeen of 24 patients with recurrent herpetic disease reported an "improvement" in terms of modification of the frequency, severity or extent of herpetic lesions. There were no important side-effects from vaccination: most patients experienced a degree of minor local reaction at the vaccination site for 24-72 hours and 3 patients reported a mild transient "flu-like" syndrome which disappeared within 48 hours of vaccination. It is intended to subject these data to the scrutiny of a placebo-controlled trial with an objective system of clinical assessment.