W e previously reported the identification of highly conserved homologous regions located in the carboxy terminus of the HIV I gp41-envelope (aa 837-844), and the amino-terminal of the beta chain of all human HLA class II antigens (aa 19-25). Murine monoclonal antibodies, raised against synthetic peptides from these homologous regions, bound not only to the isolated peptides, but also to the native gp160 and class II molecules. In this study one-third of sera from HIV I-infected individuals, at different disease stages, were found to react with both the gp41 and class II-derived peptides. These sera also reacted with "native" HLA class II molecules. The potential affects of such autoantibodies on normal immune functions were examined. It was found that in the presence of class II-cross-reactive (but not control) sera, the proliferative responses of normal CD4+ T cells to tetanus toxoid and allogeneic stimuli were markedly decreased. In addition, these sera could eliminate class II-bearing cells by antibody dependent cellular cytotoxicity. Similar affects were seen with affinity-purified IgG antibodies from patients' sera. Thus, the "molecular mimicry" between HIV I and HLA class II antigens, may lead to the generation of autoantibodies in HIV I-infected individuals that may contribute to the early functional impairment of CD4+ T cell observed in many HIV I-infected individuals.