Melatonin has been shown to act as a radical scavenger in various chemical and biological model systems in vitro. Kinetic evidence is now provided showing that melatonin inhibits the irreversible degradation of hemoglobin (Hb), when incubated with red blood cells exposed to the oxidant activity of cumene hydroperoxide (cumOOH). A decrease of heme loss and accumulation of soluble methemoglobin (met-Hb) are explained in terms of the interaction of the indoleamine with perferryl Hb (Hb[Fe(IV)=O]), a highly reactive Hb-derived radical species responsible for the irreversible Hb degradation. A kinetic study, in pure chemical solution, showed that melatonin can effectively reduce the oxoferryl heme group of perferryl-Hb, thus forming met-Hb. The reducing activity of melatonin is of the same order as that of Trolox, the water-soluble vitamin E analog. This novel radical-scavenging activity of melatonin may contribute to the previously observed protective effects of melatonin in ischemia-reperfusion injury.