We described in a previous study two pathways by which Th cells induced macrophage membrane IL-1(mIL-1) during Ag presentation. One pathway was lymphokine-independent and mediated by cell-cell contact. The second was lymphokine-dependent. We have now characterized this lymphokine and show that it belongs to the TNF family of proteins. All T(H)1 and most T(H)2 clones produced the lymphokine, although T(H)1 levels were markedly greater. Both types of clones also induced macrophage mIL-1 by cell-cell contact (i.e., in the absence of lymphokine release). Examination of macrophages by in situ hybridization showed that both IL-1α and IL-1β mRNA were coordinately induced during Ag presentation to either T(H)1 or T(H)2 clones.