To determine the effect of prolonged suppressive acyclovir therapy on the antibody response to herpes simplex virus type 2 (HSV-2) proteins, we studied sequential sera from 33 patients with frequently recurring (six or more recurrences per year) genital herpes. Twenty-two patients received 400 mg of oral acyclovir and 11 received placebo, twice daily for one year. Sera collected at enrollment, after six months and 12 months of therapy, and during the first recurrence after cessation of therapy were evaluated by western blot for levels of antibodies to HSV-2, gB, gG, gC/gE, VP16, and gD. Mean levels declined by 27%-39% after one year of acyclovir. The magnitude of the decrease in antibody levels was not correlated with disease severity either during or after therapy. Patients with high relative antibody levels to gB after therapy had more-severe first recurrences after therapy than did patients with antibody levels to gB less than or equal to the median. Antibody levels were not restored after the first untreated recurrence.