Carbon monoxide, long considered only as a toxic gas, has recently been shown to mediate potent anti-inflammatory and other salutary effects in rodents when it is used at low doses. Carbon monoxide is one of the products of the degradation of heme by heme oxygenase 1. Until recently, these beneficial effects of carbon monoxide were shown only when it was given before a stress stimulus. Hagazi and colleagues have recently shown that this substance is effective even when it is given after a disease process has started. The effects of low doses of carbon monoxide are complemented by the production of biliverdin and probably also by ferritin, which are additional products of heme degradation.