A potential component that may be useful for passive immunotherapy for HIV-1 is human monoclonal antibodies (HumAbs) possessing potent anti-HIV-1 activity that is directed against conserved regions of the envelope glycoprotein. Such antibodies would, in principle, have the ability to neutralize diverse isolates of HIV-1. To develop such reagents, hybridomas were derived by initial Epstein Barr virus transformation of peripheral blood mononuclear cells (PBMCs) from an asymptomatic HIV-1 seropositive donor followed by fusion with heteromyelomas, and secreted anti-HIV-1 antibodies were further characterized. The specificity of one HumAb, designated as clone 3, was determined by enzyme-linked immunosorbent assay (ELISA) and Western blotting analyses that indicated reactivity to the transmembrane envelope glyco-protein gp41. Synthetic pentadecapeptides overlapping by 10 amino acids were utilized for epitope mapping of clone 3; a decapeptide GCSGKLICTT in the transmembrane gp41 was identified as the epitope. Clone 3 bound to SupT1 cells infected with HTLV-IIIB in fluorescent activated cell sorting analysis. In addition, in vitro biological assays demonstrated that clone 3 possessed neutralization reactivity against diverse laboratory isolates as well as an AZT-resistant isolate. Therefore, clone 3 reactivity defines a conserved neutralizable site on the HIV-1 transmembrane glycoprotein. Clone 3 and the conserved immunogenic epitope on gp41 could be useful in passive and active immunotherapy for the acquired immunodeficiency syndrome (AIDS).