Three virus isolates HTLV-IIIB/lyA, HTLV-IIIB/lyB and HTLV- IIIB/lyO, obtained by passaging and propagating the HTLV- IIIB/H9 isolate in three separate cultures of mixed peripheral blood mononuclear cells (PBMC) from donors of blood type A, B or O, respectively, were tested for susceptibility for virus neutralization by the monoclonal antibody (MAb) AH16 directed against the blood group A epitope. MAb AH16 was previously shown to inhibit cell-free virus infection using HTLV-IIIB propagated in H9 cells. AH16 showed a concentration-dependent inhibition of the HTLV- IIIB/lyA isolate but did not inhibit the HTLV- IIIB/lyB or the HTLV- IIIB/lyO isolate. Specificity of the MAb-mediated inhibition was shown using A-antigen (tetrasaccharide). Thus, HIV infection of PBMC from donors with blood type A appears to induce expression of host-cell-encoded carbohydrate blood group A epitope on HIV which can be a target for MAb-mediated virus neutralization.