The sera of lymphomatous and healthy hamadryas baboons of the main, lymphoma-prone Sukhumi stock were tested for antibodies reacting with HTLV-I antigens in the indirect immunofluorescence test. Antibodies of this specificity were found in all but one of 58 lymphomatous baboons and in 45% of 177 healthy ones. The prevalence of HTLV-I reactive antibodies in lymphoma-free baboon populations (including 118 Sukhumi "forest" stock animals and 195 baboons imported in 3 groups from Ethiopia) was consistently lower (5-8%). The specificity of baboon antibodies reacting with HTLV-I or a related agent is supported by the following evidence: Concordant reactivity pattern of baboon sera with several HTLV-I-positive and-negative human cell lines; elimination of baboon sera anti-HTLV reactivity by absorption with purified HTLV-I, but not by other retroviruses; significant correlation between immunofluorescence titers of baboon sera and their reactivity in enzyme-linked immunosorbent assay with purified HTLV-I; competition of baboon anti-HTLV with monoclonal antibodies GIN-14 for binding of the epitope on p19HTLV-I. The prevalence of anti-HTLV positives in the main Sukhumi stock increased by age, reaching its maximum (approx. 80%) at 5-15 years, and showed no significant sex-related variation. The level of anti-HTLV antibodies in lymphomatous baboons and in age-, sex- and population-matched healthy ones did not differ. However, in pre-lymphoma sera these antibodies reached significantly higher levels than in sera of lymphomatous baboons (obtained in the terminal stage) or of matched, healthy controls.