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Sera from 39,898 blood donors were tested for HTLV-1 antibodies using two enzyme immunoassays (EIA). Sera testing initially reactive (IR) were retested in duplicate by both EIAs. Sera testing repeatedly reactive (RR) were further tested by two Western blots (WB) and by two radioimmunoprecipitation assays (RIPA). There were 176 (0.44%) EIA IR and 68 (0.17%) RR results. On WBs, 10 of the 68 EIA RR sera demonstrated reactivity to HTLV-1 gag gene-encoded core protein p24, with or without reactivity to other core proteins (p19, p28, p53/55). These ten sera were the only ones demonstrating reactivity on RIPAs to other HTLV-1 gene products - env gene-encoded glycoproteins gp46, gp61/68, or tat gene-encoded HTLV-1 transcriptional activator p40x. These ten sera were interpreted as positive for HTLV-1 antibodies. Of the remaining 58 EIA RR sera, 21 were negative by WBs and RIPAs, 37 sera demonstrated reactivity to various combinations of p19, p28, and p53/55, but not to p24 on WBs. These 37 sera were interpreted as "indeterminate", because they were negative by RIPAs. We conclude that: 1) EIA testing and WB/RIPA verification identified 10 (0.025%) HTLV-1 infected individuals among 39,898 low-risk blood donors; 2) anti-p24 may be a more sensitive and specific indicator of HTLV-1 infection than antibodies to p19, p28, or p53/55; and 3) presently, both WB and RIPA are needed to verify HTLV-1 EIA reactivity.