A new method of hydrogen peroxide determination for the measurement of catalase activity and rates of hydrogen peroxide removal by erythrocytes was described. Hydrogen peroxide was determined by converting it to the indamine dye with a water-soluble ironporphyrin and measuring the absorbance at 590 nm. This method was applied to the assay of catalase in hemolysates from human, rat and mouse blood. The activities obtained were in agreement with those obtained by other methods including UV method. The present method was also applied to the determination of rates of hydrogen peroxide removal by intact erythrocytes from human subjects, rats and mice. Data suggested that normal erythrocytes have substantial capacity to remove extracellular hydrogen peroxide. From the measurement of catalase activity in erythrocytes treated with 3-amino-1,2,4-triazole and rates of hydrogen peroxide removal by the erythrocytes, it is deduced that rate constants related to the hemoglobin content (k/g Hb) for hydrogen peroxide removal by catalase in normal and acatalasemic erythrocytes are 42.0 +/- 6.0 and 8.0 +/- 3.0, respectively.