We have detected inhibitory antibody against HIV-1 reverse transcriptase (RT) in the sera of HTLV-1 carriers, although the lack of immunological cross reaction between HIV-RT and other mammalian retroviruses, including HTLV-1, has been reported elsewhere (1). Sera from 72 HTLV-1 carriers in the southern part of Kyushu in Japan and 61 controls from the same district were studied and controlled for age and sex. One of the 72 HTLV-1 carriers was HTLV-1 antibody-positive, while the others were HIV-1 antibody-negative by ELISA and Western blot method. Sera from two HTLV-1 carriers and one hemophiliac with combined HIV-1 and HTLV-1 infections were found to inhibit HIV-1 RT activity. The inhibition ratios were 72, 65 and 72%, respectively. Protein A effluents of inhibitory sera retained no inhibitory capacity. These results might suggest that HTLV-1 and HIV-1 reverse transcriptase may share common or similar epitopes and that antibodies raised to the HTLV-1 RT react with that of HIV-1 RT in an inhibitory mode. However, the RT inhibitory antibody-positive sera had no neutralizing capacity against HIV-1 infection, supporting the idea that HIV-1 RT inhibitory antibody may not contribute to neutralization against HIV-1. The results also suggest that previous exposure to HTLV-1 does not contribute to neutralizing antibodies against HIV-1 in HIV-1 infected individuals.