Antigen‐presenting macrophages (MΦ) were derived from day 7 cultures of bone marrow stem cells using L cell conditioned medium. The adherent bone marrow‐derived macrophages (BMMΦ) were 100% esterase‐positive, 95% positive for C3 receptors, 93% positive for Fc receptors, and 95% actively phagocytic. Indirect immunofluorescence using anti‐Ia monoclonal antibodies resulted in 60% Ia‐positive BMMΦ on day 7 of stem cell culture. BMMΦ could stimulate mixed lymphocyte reaction (MLR) proliferation across an I‐A subregion difference, but not across I‐J subregion differences. This contrasted with splenic MΦ which stimulated MLR proliferation across both an I‐A and I‐J subregion difference. The apparent lack of I‐J subregion determinants on BMMΦ correlated with their ability to function as antigen‐presenting cells. In these experiments, BMMΦ effectively reconstituted the trinitrophenyl‐specific IgM plaque‐forming cell (PFC) response of B cells but not the primary burro red blood cell (BRBC)‐specific IgM‐PFC response of MΦ‐depleted spleen cells. When BMMΦ were added to BRBC‐primed T and B cells, they reconstituted the secondary IgG PFC response to levels obtained using splenic MΦ. These experiments relate the differential expression of H‐2I region determinants on antigen‐presenting cells with their functional capacity.