Sera from 75 children from an isolated subsistence farming region of the Ugandan Nile valley in 1972-1973 showed a unique pattern of antibody titer to HTLV-III: a high prevalence but low titer for a limited number of viral proteins. The sera were originally collected at random as controls for a study of Burkitt's lymphoma. Mean age was 6.4 years. Sera were tested quantitatively by ELISA and 50 of 55 positives were confirmed by Western blot. The most prominent bands had molecular weights of 76,000, 41,000 and 24,000, coinciding with HTLV-III antigens previously described. The geometric mean titer was 295 with a range of 100-1000. The results suggest high prevalence of a closely related virus in this population.