During 1986 sera from 2508 individuals representing various groups of healthy subjects and patients in Dar es Salaam (the capital city of Tanzania), Bukoba (the capital of Kagera region in the northwest corner of Tanzania), Arusha (in the northeast of Tanzania) and Mbeya (in the southwest of Tanzania) were screened for antibodies to HIV by enzyme-linked immunosorbent assay (ELISA). All ELISA-positive sera were also tested by Western blot analysis. In Dar es Salaam HIV antibodies were demonstrated in 3.6% of 192 pregnant women, 5.2% of 784 blood donors, 29.0% of 224 barmaids, 8.0% of 50 male bar workers, 9.25% of 400 male and 12.2% of 90 female patients attending a clinic for sexually transmitted diseases (STDs), 85.7% of 35 patients with herpes zoster and in 97.6% of 84 patients clinically suspected of AIDS. Among the barmaids the seropositivity rate was higher in younger women (45%) than in middle-aged women (11%). Only three (4.6%) out of 65 HIV-seropositive barmaids had HIV-related symptoms. The prevalence of HIV seropositivity among healthy low-risk subjects was highest in Bukoba, namely 16% of 100 pregnant women and 13.9% of 36 blood donors, while in Arusha only one (0.7%) of the 144 pregnant women and none of 41 bar workers, none of 42 blood donors and none of 61 patients with STD were positive. In Mbeya, 3.4% of 118 pregnant women and 11.8% of 34 men with STD were seropositive. Thus the prevalence of HIV infection differs considerably in various population groups and in various parts of Tanzania.