Bilirubin is generally considered to be a diagnostically useful, sometimes toxic, metabolic waste product--and nothing more. Many studies, however, summarized in this article, have shown that bilirubin is an effective lipid-soluble antioxidant in vitro, even at physiologic concentrations, vying with even vitamin E in its ability to intercept and inhibit free radical chain reactions that generate hazardous lipid peroxides. These studies suggest that bilirubin may have a biochemical function in vivo and belong to a group of low-molecular weight antioxidants that together provide protection from cellular damage by endogenous-organic free radicals. Dovetailing with the notion that bilirubin may have a protective function is the recent discovery that heme oxygenase, one of the enzymes responsible for bilirubin formation, is a heat-shock protein, one of a group of proteins that are thought to protect organisms from oxidative and other forms of biochemical stress. Thus, the biochemical path from red to green to yellow may defend as well as degrade, and modest levels of the end product may possibly be physiologically beneficial.