We studied the relations between the cellular immune response, pre-existing complement-fixing antibody and virus type with duration of virus excretion in genital herpes simplex virus (HSV) infection. Thirty-six patients (seven with HSV-1 and 29 with HSV-2) with genital herpes underwent serologic testing, sequential viral cultures and weekly determination of lymphocyte-transformation stimulation index with inactivated HSV antic n. The duration of virus excretion was shortest in those with pre-existing complement-fixing antibody, was unrelated to virus type, and was inversely correlated with the magnitude of the mean peak stimulation index (r = -0.69, P less than 0.001). Prolonged virus excretion occurred in patients with a delayed and diminished peak index. Recurrent episodes had a higher peak index (29.4 compared to 14.5) (P less than 0.02), an earlier development of the peak during recurrences (9.1 vs. 25.8 days) (P less than 0.01) and a briefer duration of viral shedding than initial episodes. Thus, the temporal course and magnitude of the stimulation index correlate with and may determine the duration of genital HSV infection.