We have studied the role of T cells in the induction of a membrane-associated form of interleukin 1 (mIL 1) in murine macrophages. T helper cell clones and a T cell hybridoma induced macrophages to express mIL 1 after an antigen-specific, Ia-restricted interaction. Induction of mIL 1 was proportional to antigen concentration and was increased in the early course of the response in macrophages pretreated in culture with interferon-γ. mIL 1 activity was detectable 4 hr after interaction with T cells. mIL 1 induction was inhibited by antibodies to either class II molecules or the T cell receptor. Two pathways of T cell-mediated mIL 1 induction could be defined. In the first, T cells, whose protein synthesizing capacity was completely eliminated by pretreatment with the irreversible protein synthesis inhibitor emetine, induced levels of mIL 1 expression indistinguishable from controls. In the second, T cells stimulated by paraformaldehyde-fixed macrophages in the presence of concanavalin A or antigen secreted a soluble factor that induced macrophage mIL 1 expression. Thus, it appears that T cells may induce macrophages to express mIL 1 both by direct cell-cell contact mediated through binding of T cell receptor to the Ia/antigen complex, and through the release of a lymphokine after activation. This lymphokine does not appear to be IL 2, IFN-γ, BSF-1, or CSF-1.