Colony-stimulating factor-induced bone marrow (BM) cultures are a good source of antigen-presenting macrophages. However, they failed to present antigen to T cell clones when antigen was introduced as a short pulse only. Adding interleukin 1 (IL1) to BM culture cells before antigen pulse restored their antigen-presenting activity concomitant to a 2-10-fold increase in Ia antigenicity. We performed a series of experiments to test the mechanism of this IL1-induced activation. Our findings suggest that IL1 influences Ia expression and antigen-specific T cell proliferation by inducing interferon-gamma (IFN-gamma) production. IFN-gamma is produced in this system probably by residual Thy-1-positive cells in the BM cell culture.