Antibodies of individuals infected with the human immunodeficiency virus type 1 (HIV-1) were used to probe the antigenicity of the HIV-1 transmembrane protein of 41 kD (gp41) by antibody-reactive peptide scanning (Pepscan). Eleven distinct sequential antibody-binding sites were defined by testing reactivity to 339 overlapping nonapeptides spanning the complete gp41 amino acid sequence. Such analysis only maps continuous antibody-binding sites of nine amino acids in length and does not identify putative discontinuous or assembled epitopes. Three B cell epitopes (aa 609–622; aa 655–699; aa 664–681) at the amino-terminal border of the putative transmembrane anchor and two (aa 732–748; aa 744–762) at the carboxyl-terminal border of this domain were the most antigenic. One antibody-binding domain (aa 834–852) with four amino acids homologous to the beta-1 domain of HLA class II beta-chain was recognized by the serum in 1 of 4 AIDS patients tested and not by any of the eight sera from symptom-free individuals. Although functional domains of gp41 involved in virus replication, cytopathicity and possibly immunosuppression were shown to bind antibodies of HIV-1-infected individuals, no relationship between antibody recognition patterns and disease progression w