Human peritoneal macrophages from healthy females have been investigated for their capability to produce interleukin-1 (IL-1), their expression of HLA-DR and -DQ, and for their antigen-presenting capacity in concanavalin A, tetanus toxoid (TT), and autologous T-cell proliferative responses. Fifteen out of thirty macrophage populations produced IL-1 but the activity was 1/5 to 1/10 that of peripheral blood mononuclear cells stimulated under similar conditions. High levels of HLA-DR were expressed on all macrophages while lower and more variable levels of DQ were found. All macrophages induced mitogen-dependent T-cell proliferation while the ability to induce a proliferative response to TT was variable, 12/23 tests were positive. In five samples stimulatory capacity of macrophages in the absence of TT was as strong as in the presence of the stimulus, suggesting that in vivo processed immunogen could be responsible for the proliferative response. The surface density of HLA-D-region-determined antigens was not indicative of the macrophages' ability to induce antigen-specific proliferation. IL-1 production, however, correlated with this function. Antigen presentation was not confined exclusively to peritoneal populations consisting of recently immigrant monocyte-like cells, nor were all young macrophages able to present antigen. This may reflect on the diversion of macrophage function by the local environment.