We have examined the susceptibility to acyclovir and frequency of acyclovir-resistant viruses in herpes simplex virus type (HSV) 2 clones isolated directly from genital lesions of 11 patients who had taken suppressive therapy (200 mg/day) for 1-9 years and 15 patients naive to acyclovir. Suppressive therapy significantly reduced the incidence of recurrence and the severity of the skin lesions. HSV samples from genital lesions were directly inoculated into Vero cells, and viral clones were isolated in the absence and presence of 10 microg/ml acyclovir. Five-hundred-and-ninety-two clones, isolated in the absence of acyclovir, were subjected to the acyclovir susceptibility test, and 155 clones isolated in the presence of acyclovir were analysed for the mechanisms of resistance to acyclovir. There were no significant differences in the susceptibility to acyclovir, the frequency of acyclovir-resistant virus and the ratio of thymidine kinase-deficient viruses in acyclovir-resistant viruses between the two groups. The frequency of acyclovir-resistant clones was about three per 10000 plaque forming units (PFU), and genital lesions contained up to 3x10(6) PFU of replicating virus in the specimens from the patients with genital herpes with or without acyclovir-suppressive therapy. Thus, the low dose of acyclovir suppressive therapy did not affect the susceptibility to acyclovir or increase the frequency of acyclovir-resistant viruses in the genital lesions.