We have previously reported that transforming growth factor-beta 1 (TGF- beta 1) inhibits interleukin-6 (IL-6) induction by IL-2 and IL-1 in fresh human monocytes. We investigated the effects of TGF-beta 1 on the expression of tumoricidal activity induced by IL-2 or interferon-gamma (IFN-gamma) in human monocytes. We showed that TGF-beta 1 specifically inhibited, in a dose-dependent manner, IL-2-induced but not IFN-gamma- induced monocyte tumoricidal activity. The inhibitory effects of TGF- beta 1 on IL-2-activated monocytes were not caused by down-modulation of the IL-2 receptor beta (IL-2R beta) because the treatment of monocytes with IL-2 and TGF-beta 1 increased IL-2R beta mRNA expression. However, we found that TGF-beta 1 down-modulated IL-2- induced IL-2R gamma mRNA, which may be responsible for the TGF-beta 1 inhibition of monocyte activation by IL-2. The resistance of the IFN- gamma-induced activation to the inhibitory effects of TGF-beta 1 could be caused by the ability of IFN-gamma to decrease TGF-beta 1 receptor expression, as shown by cross-linking experiments. Overall, these results showed that TGF-beta 1 is a powerful inhibitor of IL-2- but not of IFN-gamma-induced activation of monocytes to a cytotoxic stage. This differential effect may be attributed to modulation of cytokine receptor expression.