We examined the effect of transforming growth factor-beta (TGF-beta) alone and in combinations with other factors on the growth and differentiation of the human promyelocytic cell line HL60 and the human monoblastic cell line U937. Treatment with TGF-beta alone did not significantly affect growth or differentiation of HL60 cells, while it significantly inhibited proliferation and induced monocytic differentiation of a small percentage of U937 cells. Combinations of TGF-beta and tumor necrosis factor-alpha (TNF-alpha) acted in synergy to inhibit cell proliferation and to induce monocytic differentiation of both HL60 and U937 cells. In contrast, no synergy was observed when HL60 cells were treated with TGF-beta in various combinations with interferon-alpha (IFN-alpha), interferon-gamma (IFN-gamma), and retinoic acid. Examination of TNF-alpha receptor expression on HL60 and U937 cells showed that these cell lines expressed comparable levels of high-affinity TNF-alpha binding sites. Treatment of HL60 and U937 cells with TGF-beta did not induce significant changes in TNF-alpha receptor expression in either cell line. In contrast, HL60 cells expressed much lower levels of TGF-beta receptors than did U937 cells. Treatment of both HL60 and U937 cells with TNF-alpha induced a dose-dependent increase in expression of TGF-beta receptors, suggesting that the synergy between TNF-alpha and TGF-beta may result, at least in part, from upregulation of TGF-beta receptor expression by TNF-alpha.