A bias to either cell-mediated or antibody-mediated effector mechanisms is induced in an immune response against a pathogen, if activated T helper cells (Th) predominantly express Th1 [interleukin (IL)-2, interferon (IFN)-gamma, and tumor necrosis factor (TNF)-beta] or Th2 (IL-4, IL-5, IL-6 and IL-10) cytokines. Here we provide evidence that, due to the capability to secrete IL-1, macrophages, but not B cells, as antigen-presenting cells (APC) induce production of IFN-gamma in resting Th cells. Normal murine splenic Th cells were activated in vitro with the superantigen Staphylococcus aureus enterotoxin B (SEB) presented by macrophages as compared to other APC from murine spleen. As determined by immunofluorescence, Th cells producing IL-2 but almost none producing IL-4 and IL-5 are generated, irrespective of the type of APC. Generation of IFN-gamma-producing Th cells is largely dependent on presentation of SEB by macrophages. The requirement for macrophages, however, is overcome if IL-1 is provided. Expression of IFN-gamma by Th cells is not induced, if production of IL-1 by macrophages is inhibited by IL-10. Our results suggest a functional dichotomy of APC: normal resting Th cells differentiate into IL-2 and IFN-gamma secreting cells (Th1 cells) if antigen is presented by macrophages, whereas presentation by B cells generates Th cells secreting IL-2, which might differentiate into Th2 cells upon re-stimulation.