S epsis is characterized by a systemic response to severe infection. Although the inflammatory phase of sepsis helps eradicate the infection, it can have detrimental consequences if left unchecked. Therapy directed against inflammatory mediators of sepsis has shown little success and has the potential to impair innate antimicrobial defenses. Heme oxygenase-1 (HO-1) and the product of its enzymatic reaction, CO, have beneficial antiinflammatory properties, but little is known about their effects on microbial sepsis. Here, we have demonstrated that during microbial sepsis, HO-1–derived CO plays an important role in the antimicrobial process without inhibiting the inflammatory response. HO-1–deficient mice suffered exaggerated lethality from polymicrobial sepsis. Targeting HO-1 to SMCs and myofibroblasts of blood vessels and bowel ameliorated sepsis-induced death associated with Enterococcus faecalis, but not Escherichia coli, infection. The increase in HO-1 expression did not suppress circulating inflammatory cells or their accumulation at the site of injury but did enhance bacterial clearance by increasing phagocytosis and the endogenous antimicrobial response. Furthermore, injection of a CO-releasing molecule into WT mice increased phagocytosis and rescued HO-1–deficient mice from sepsis-induced lethality. These data advocate HO-1–derived CO as an important mediator of the host defense response to sepsis and suggest CO administration as a possible treatment for the disease.