Induction of heme oxygenase-1 (HO-1) is protective in tissue injury in models of allograft rejection and vascular inflammation through either prevention of oxidative damage or via immunomodulatory effects. To examine the specific role of HO-1 in modulating the immune response, we examined the differences in immune phenotype between HO-1 knockout (HO-1^−/−) and wild-type (HO-1^+/+) mice. Consistent with previous findings, marked splenomegaly and fibrosis were observed in HO-1^−/− mice. The lymph nodes of HO-1-deficient mice demonstrated a relative paucity of CD3- and B220-positive cells, but no such abnormalities were observed in the thymus. Flow cytometric analysis of isolated splenocytes demonstrated no differences in the proportions of T lymphocytes, B lymphocytes or monocytes/macrophages between the HO-1^−/− and HO-1^+/+ mice. Significantly higher baseline serum IgM levels were observed in HO-1^−/−versus HO-1^+/+ mice. Under mitogen stimulation with either lipopolysaccharide or anti-CD3/anti-CD28, HO-1^−/− splenocytes secreted disproportionately higher levels of pro-inflammatory Th1 cytokines as compared to those from HO-1^+/+ mice. These findings demonstrate significant differences in the immune phenotype between the HO-1^−/− and the HO-1^+/+ mice. The absence of HO-1 correlates with a Th1-weighted shift in cytokine responses suggesting a general pro-inflammatory tendency associated with HO-1 deficiency.