AbstractTransforming growth factor‐β1 (TGF‐β1) mediates many immunosuppressive effects on immune cells and can inhibit the production of tumor necrosis factor and interleukin 1 (IL 1). However, TGF‐β1 can stimulate the production of IL6 and platelet‐derived growth factor, indicating that TGF‐β1 initiates complex effects on the production of cytokines. In this report we show that treatment of peripheral blood monocytes with TGF‐β1 leads to the induction of a recently described IL1 receptor antagonist protein (IRAP). The effect of TGF‐β was both dose and time dependent. TGF‐β1 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‐β1 are mediated.