Many viruses, including retroviruses, are characterized by their specific cell tropism^1–3. Lymphadenopathy-associated virus (LAV) is a human lymphotropic retrovirus isolated from patients with acquired immune deficiency syndrome (AIDS) or related syndromes^4,5, that displays selective tropism for a subset of T lymphocytes defined by the expression of a surface glycoprotein of relative molecular mass 62,000 (62K) termed T4 (refs 6–8). This glycoprotein delineates a subset of T lymphocytes with mainly helper/inducer functions, while T lymphocytes of the reciprocal subset express a glycoprotein termed T8, have mainly cytotoxic/suppressor activities, and are unable to replicate LAV^7,9. Such a tropism may be controlled at the genomic level by regulatory sequences, as described for the human T-cell leukaemia viruses HTLV-I and -II (refs 2,3). Alternatively or concomitantly, productive cell infection may be controlled at the membrane level, requiring the interaction of a specific cellular receptor with the virus envelope, as demonstrated recently for Epstein–Barr virus (EBV)¹. Therefore, we have investigated whether the T4 molecule itself is related to the receptor for LAV. We report here that preincubation of T4⁺ lymphocytes with three individual monoclonal antibodies directed at the T4 glycoprotein blocked cell infection by LAV. This blocking effect was specific, as other monoclonal antibodies—such as antibody to histocompatibility locus antigen (HLA) class II or anti-T-cell natural killer (TNK) target—directed at other surface structures strongly expressed on activated cultured T4⁺ cells, did not prevent LAV infection. Direct virus neutralization by monoclonal antibodies was also ruled out. These results strongly support the view that a surface molecule directly involved in cellular functions acts as, or is related to, the receptor for a human retrovirus.