Bone marrow cells from C3H/HeJ mice were cultured in recombinant granulocyte-macrophage-CSF (rGM-CSF) or recombinant or purified (natural) preparations of macrophage-CSF (CSF-1) for 7 days, the adherent macrophages removed enzymatically, recultured in the absence of growth factor, and examined for their differentiative and functional characteristics. Expression of Ia Ag differed markedly in these two populations. Antibody plus complement-mediated cytotoxicity indicated that the percent of Ia-positive macrophages was low (approximately 15 to 20%) in both populations. Treatment of cultures with rIFN-gamma increased the percentage of Ia-positive cells in both populations; however, more CSF-1-derived macrophages were induced to become Ia positive than rGM-CSF-derived macrophages. In contrast, total Ia expression and Ia density per cell, as measured by ELISA and quantitative immunofluorescence, respectively, showed that medium-treated rGM-CSF-derived macrophages exhibited greater total Ia expression and higher Ia density per cell than CSF-1-derived macrophages. Treatment of CSF-1-derived macrophages with rIFN-gamma increased total Ia expression per culture to the levels exhibited by rGM-CSF-derived cells, although the density of Ia Ag per cell remained lower than in rGM-CSF-derived cells (medium or rIFN-gamma-treated). Northern blot and slot blot analysis of cytoplasmic RNA extracted from freshly harvested rGM-CSF- or CSF-1-derived cells indicated that the differences seen in basal Ia expression were also reflected at the level of steady-state, Ia-specific mRNA. The functional capacity of these two macrophage populations to stimulate Ag-specific T cell proliferation was also assessed. rGM-CSF- or CSF-1-derived macrophages were first cultured in the absence or presence of rIFN-gamma, Ag-pulsed, and irradiated before co-culture with Ag-primed, purified T cells. Ag-induced T cell proliferation was significantly greater in rGM-CSF- than in CSF-1-derived macrophages. Treatment of either population with rIFN-gamma had only a minimal effect on the ability of either macrophage population to stimulate Ag-specific T cell proliferation. These findings suggest that the development of mature macrophages from bone marrow progenitors under the influence of either GM-CSF or CSF-1 may, in part, underlie the functional heterogeneity of different macrophage populations.