We have raised a rabbit monospecific antibody against a synthetic peptide derived from a sequence within the COOH-terminal portion of the reverse transcriptase (RT) of HIV-1. This sequence was also found to be conserved in the predicted amino acid sequence of HIV-2. The antibody, designated C2003, cross-reacted with HIV-2 RT on immunoblots of HIV-2 virus extract and directly inhibited HIV-2 RT activity. Fractionation of HIV-2 RT by immunoaffinity chromatography with C2003 antibody yielded a pair of viral proteins of 68 and 55 kD associated with both RT and RNAse H activities. Both proteins were found to be highly immunogenic, recognized by 11 of 11 human sera that previously tested positive for antibodies to HIV-2 antigens in immunoblot assays.