An individual serum stored since 1959 from Central Africa has been demonstrated reactive against HTLV-III by ELISA, Western blotting, and by immunoprecipitation against HTLV-III as well as STLV-III. This serum was from a set of 1213 plasmas from various parts of Africa, 818 dating from 1959. Because of the known high false positive rates on Abbott EIA in long-term frozen sera, those found to be above the cutoff using the formula (mean of 3 negative serum controls plus 10% of the mean of 2 positive serum controls) x 3 were retested by immunofluorescence microscopy and Western blot enzymatic immunoassay. 1 serum taken from Central Africa in 1959 was positive by both tests. This plasma had an OD more than 7 x the cutoff value. It reacted strongly in immunofluorescence with infected H9 cells. Reactivity was detected by Western Blot against all major HTLV-III viral proteins and polypeptide 121. By immunoprecipitation it reacted with all major gag and env encoded proteins of HTLV-III and the gag encoded proteins of STLV-III-AGM. These results suggest that HTLV-III prevalence was low in 1959, but that at least 1 individual was exposed to a similar virus over 25 years ago in Central Africa.