Heme oxygenase (HO)-1 catabolizes heme into three products: carbon monoxide (CO), biliverdin (which is rapidly converted to bilirubin) and free iron (which leads to the induction of ferritin, an iron-binding protein). HO-1 serves as a "protective" gene by virtue of the anti-inflammatory, anti-apoptotic and anti-proliferative actions of one or more of these three products. Administration of CO, biliverdin, bilirubin or iron-binding compounds is protective in rodent disease models of ischemia-reperfusion injury, allograft and xenograft survival, intimal hyperplasia following balloon injury or as seen in chronic graft rejection and others. We suggest that the products of HO-1 action could be valuable therapeutic agents and speculate that HO-1 functions as a "therapeutic funnel", mediating the beneficial effects attributed to other molecules, such as interleukin-10 (IL-10), inducible nitric oxide synthase (NOS2; iNOS) and prostaglandins. This Review is the third in a series on the regulation of the immune system by metabolic pathways.