Transforming growth factor-beta 1 (TGF-beta 1) mediates many immunosuppressive effects on immune cells and can inhibit the production of tumor necrosis factor and interleukin 1 (IL 1). However, TGF-beta 1 can stimulate the production of IL 6 and platelet-derived growth factor, indicating that TGF-beta 1 initiates complex effects on the production of cytokines. In this report we show that treatment of peripheral blood monocytes with TGF-beta 1 leads to the induction of a recently described IL 1 receptor antagonist protein (IRAP). The effect of TGF-beta 1 was both dose and time dependent. TGF-beta 1 induced de novo synthesis of IRAP, as Northern blotting experiments indicated a rapid and transient induction of the mRNA encoding IRAP. The induction of IRAP suggests a potential mechanism by which some of the inhibitory effects of TGF-beta 1 are mediated.