Murine monoclonal antibodies to monomorphic components of HLA-DR antigen were used to analyze the distribution and function of DR molecules on non-T mononuclear leukocytes from peripheral blood. On the basis of indirect immunofluorescence and complement-mediated cytolysis. DR antigen was detected on approximately 70% of non-T cells (DR+) and could not be detected on approximately 30% of non-T cells (DR-). A fluorescence-activated cell sorter was used to separate non-T cells into DR+ and DR- populations, and each population was studied. At least one-third of DR- cells were monocytes, and the remainder were surface-immunoglobulin-negative lymphocytes. Analysis of [35S]methionine-labeled proteins by the method of two-dimensional polyacrylamide gel electrophoresis indicated that DR+, but not DR-, cells biosynthesize DR molecules DR+ cells stimulated strongly in the autologous and allogeneic mixed lymphocyte reactions (MLR) and supported T cell proliferation to soluble antigens, whereas DR- cells stimulated in the allogeneic MLR but failed to stimulate in the autologous MLR or to support T cell proliferation to soluble antigens. When present continuously in culture, one monoclonal anti-DR antibody (antibody 2.06) modestly inhibited T cell proliferative responses. Another antibody (antibody 1.35) markedly enhanced the autologous MLR and the proliferative response to soluble antigens, but had no effect on the allogeneic MLR. These data suggest that DR+ and DR- non-T populations are functionally distinct, and that DR antigen may be required for presentation of soluble antigen and stimulation in the autologous MLR. Antigens in addition to DR may stimulate allogeneic T cell proliferation.