Eighteen asymptomatic men with persistent human immunodeficiency virus type I (HIV-I) p24 antigenaemia were treated with zidovudine 250-500 mg (+/- acyclovir 800 mg) 6-hourly for 4-12 weeks, and subsequently with zidovudine 500 mg (+/- acyclovir 1600 mg) 12-hourly for 36 weeks. After 24 weeks six additional HIV antigenaemic subjects were entered and treated directly with zidovudine 500 mg 12-hourly. Over the treatment period serum HIV-I p24 (HIV-Ag) levels declined in all 24 subjects; significantly so in 17, and to below cut-off values in five. Mean serum HIV-Ag levels in different treatment groups declined in 68-78%. Initial increases in CD4+ cell counts were not sustained. Over 48 weeks serum HIV-Ag levels rose in three out of five non-treated men with persistent HIV antigenaemia, and they slightly declined in two; the mean serum HIV-Ag level in this group rose 67%. Regression of enlarged lymph nodes was seen in 19 out of 19 of the zidovudine-treated subjects. In the 24 zidovudine-treated subjects no disease progression occurred during follow-up, whereas two out of five non-treated men went on to develop CDC group IV A, and IV C-2 disease, respectively. Adverse reactions to the study drugs were infrequent and mild. Anaemia caused symptoms in two, but serious leucopenia or neutropenia was not observed. An initial positive effect on thrombocyte numbers was not sustained. These data demonstrate that in asymptomatic HIV-infected subjects zidovudine 500 mg 12-hourly is well tolerated and has a persistent inhibitory effect on viral replication.