Hydrogen peroxide removal rates by hemoglobin were enhanced in the presence of reduced pyridine nucleotides. The species which had the activity to oxidize pyridine nucleotides was purified from human blood and identified as hemoglobin A. Hydrogen peroxide removal rates by hemoglobin A without reduced pyridine nucleotides at 0.2 mM hydrogen peroxide were 0.87+/-0.11 micromol/s/g hemoglobin, and the removal rates using 0.2 mM NADH and NADPH were 2.02+/-0.20 and 1.96+/-0.31 micromol/s/g hemoglobin, respectively. We deduced that the removal reaction by hemoglobin included formations of methemoglobin and the ferryl radical and reduction of the latter with pyridine nucleotides. The hydrogen peroxide removal ability by hemoglobin was less than that by catalase but was larger than that by glutathione peroxidase-glutathione reductase system at 0.2 mM hydrogen peroxide. Under acatalasemic conditions, it was suggested that NAD(P)H were important factors to prevent the oxidative degradation of hemoglobin.