Luminol-enhanced chemiluminescence, initially induced by exogenous H₂O₂ or enzymatic reactions which generate reactive oxygen species in situ, is inhibited by reduced pterins. Except dihydroneopterin which is as effective as tetrahydroneopterin, all tetrahydropterins are more effective than dihydropterins, which in turn is more effective than oxidized pterins in removing reactive oxygen species such as O^∸₂ and H₂O₂. The antioxidant efficacy of some reduced pterins is better than ascorbic acid and L-glutathione. Reduced pterins may act as physiological scavengers for reactive oxygen species, and play an important role in cellular defense against oxidative damage.