Biliverdin and bilirubin are reducing species and hence potential antioxidants formed by the action of heme oxygenase and biliverdin reductase. Indeed, there is increasing evidence for the suggestion that a beneficial role of the potentially toxic bilirubin may be to act as a powerful chain-breaking antioxidant in biological systems, and that bilirubin may contribute to the cellular and tissue protection seen with increased heme oxygenase. This article reviews the in vitro antioxidant activities of the two bile pigments with emphasis on the different physiological forms of bilirubin and types of oxidants, and discusses these properties in light of the presence and reactivity other nonproteinaceous antioxidants.