In this study we show that bone marrow macrophages (BMM phi), derived by culturing bone marrow stem cells in macrophage colony-stimulating factor (M-CSF)-containing medium, and activated by an optimal dose of interferon-gamma, selectively interacted with only some out of a group of protein antigen-specific T cell clones as measured by antigen-specific T cell proliferation. Antibody inhibition experiments employing monoclonal anti-CD4 antibodies suggest that the failure of various T cell lines to cooperate with BMM phi might be due to a low avidity of the interaction between these T cells and the accessory cells. We further show that BMC that were allowed to mature in the presence of recombinant granulocyte-macrophage colony-stimulating factor (GM-CSF) developed into highly efficient accessory cells leading to antigen-specific activation of all T cell clones tested. No correlation was found with the level of expression of MHC class II genes induced in GM-CSF-treated BMM phi, although significant amounts of transcripts of A alpha, A beta and of the non MHC-encoded invariant gamma-chain were detected by Northern blot analysis.