The role of sexual transmission of hepatitis C virus in Black South Africans was evaluated by a seroprevalence study of sentinel populations at varying risk for sexually transmitted diseases (STD). Prevalence of anti-hepatitis C virus antibodies was found to be 1.8% in an STD clinic sample of 272, 0.7% in a family planning sample of 148, 3.3% in a sample of 246 'blue collar' workers (81% of rural origin), and 0.9 in a sample of 117 new blood donors. All samples were from Black adults. The differences between them were not significant (P = 0.2348). In contrast, the prevalence of anti-human immunodeficiency virus antibodies in the STD sample (5.5%) was statistically significantly different (P = 0.00095) from the family planning clinic sample (1.4%) and the blue collar sample (0.8%) as well as from the reported prevalence for black blood donors in the Johannesburg area (0.7%). No evidence supporting a role for sexual transmission of hepatitis C virus was found, while the prevalence of infection appeared to be higher in rural populations and in males. These features are similar to hepatitis B in this population.