Purified T lymphocytes (E rosetting cells) isolated from the involved lymphoid organs (lymph nodes and spleen) of five patients with Hodgkin's disease (HD) were cloned under culture conditions (phytohemagglutinin plus interleukin-2) that allow clonal expansion of most T lymphocytes. A total number of 104 CD4+ T cell clones so obtained were tested for their ability to proliferate in response to autologous mitomycin-treated non-T cells. About half of these clones but none of 234 CD4+ T cell clones derived from normal lymphoid tissues or peripheral blood displayed a proliferative response to autologous stimulators. When clones proliferating in autologous mixed lymphocyte reaction (AMLR) were assessed for their ability to respond in allogeneic MLR (allo-MLR), most of them were found to exhibit consistent proliferation in response to more than one haplotype. Both the AMLR and the allo-MLR by HD clones were inhibited by adding monoclonal antibodies (MoAbs) reactive with monomorphic determinants of major histocompatibility complex (MHC) class II (DR) antigens to the cultures, whereas MoAbs reactive with MHC class I antigens were without effect. These studies suggest that lymphoid organs involved by HD contain high proportions of CD4 T cells showing abnormal recognition of DR antigens. These unusual cells may play an important role in the pathogenetic mechanisms occurring in HD.