AbstractA naturally occurring receptor‐level antagonist of interleukin‐1 (IRAPorIL‐1 ra) has recently been cloned. To determine what stimuli might regulate this inhibitor, cytokines were tested for their effects on the steady‐state level of IRAP mRNA in phorbol ester‐differentiated U937 cells. The cytokines tested fell into one of three groups: (a) inducers: granulocyte‐macrophage colony‐stimulating factor (GM‐CSF), IL‐4, (b) weak inducers (<2‐fold stimulation): [IL‐lα, IL‐Iγ, and transforming growth factor‐β (TGF‐γ)] and (c) cytokines with no effect: (IL‐2, platelet‐derived growth factor, acidic fibroblast grouth factor, basic fibroblast growth factor, epidermal growth factor, granulocyte colony‐stimulating factor, IL‐3, IL‐5, IL‐6, interferon‐y, multi‐colony stimulating factor, tumor necrosis factor ‐a and IRAP itself. One hundred U/ml of either GM‐CSF or IL‐4 was the dose inducing peak IRAP mRNA expression; that peak expression occurred 12 h after addition of cytokine. GM‐CSF induced a 34 ±15‐fold increase in IRAP mRNA, and IL‐4 induced a 15± 6‐fold increase. In the same RNA samples, GM‐CSF increased IL‐ip mRNA 5.9 ± 1.7‐fold, but IL‐4 decreased IL‐Iγ mRNA to half that of control levels (0.45 ± 0.17). Thus, a single stimulus (IL‐4) decreased the expression of an agonist (IL‐1) while it increased the expression of an antagonist (IRAP). When U937 cells were treated with both IL‐4 and GM‐CSF, the level of IRAP mRNA induced was additive, suggesting that the cytokines acted differently to increase IRAP mRNA levels. The level of IL‐1 mRNA in cells treated with both IL‐4 and GM‐CSF was intermediate. Dexamethasone and cycloheximide inhibited all mRNA increases and did not reverse IL‐4‐induced decreases in IL‐1 mRNA. These studies have identified two cytokines which induce IRAP in the monocytic cells studied, and have partially characterized the differential regulation of IL‐1 and its antagonist, IRAP.