Heme oxygenase-1 (HO-1) is thought to contribute to host defense reactions against various stresses. In addition, recent reports have suggested that HO-1 modulates immunocyte activation and functions. HO-1 suppresses mast cell degranulation, but whether HO-1 suppresses cytokine synthesis as well is not yet known. We examined whether rat HO-1 cDNA transfected rat basophilic leukemia (RBL)-2H3 cells have altered cytokine production in response to stimulation with anti-ovalbumin (OA) serum/OA compared to Mock transfected RBL-2H3 cells. HO-1 inhibited anti-OA serum/OA-induced IL-3 and TNF-alpha production. Inhibition of HO-1 activity by Zn (II) protoporphyrin IX, a specific HO-1 inhibitor, prevented the suppression of TNF-alpha production. The cytokine inhibition by HO-1 was associated with selective suppression of the DNA-binding activity of AP-1 transcription factors. The suppression of mast cell cytokine production by HO-1 may be an important aspect of the processes that lead to resolution of allergic inflammation.