The ferric and ferrous forms of bovine erythrocyte green hemeprotein react with hydroperoxides to form higher oxidation state intermediates with absorbance maxima in the Soret region at 426 and 422 nm, respectively. In the absence of an appropriate reductant, these intermediates undergo rapid bleaching reactions. 2,2'-Azinobis(3-ethylbenzthiazoline-6-sulfonic acid) rapidly reduces the intermediate formed by reaction of ferric green hemeprotein with hydrogen peroxide, thereby preventing bleaching and allowing the rate of the intermediate formation to be calculated from the coupled dye oxidation. This rate constant of 70 M-1 s-1 at 23 degrees C is similar to those determined by bleaching and by direct photometric detection of the intermediate. Dihydroriboflavin rapidly reduces the intermediate formed by reaction of ferrous green hemeprotein with hydrogen peroxide, thereby preventing bleaching and allowing the rate of the intermediate formation to be calculated from the coupled dihydroriboflavin oxidation; the rate constant of 2 x 10(4) M-1 s-1 at 23 degrees C is similar to the value calculated by direct detection of the intermediate. The results demonstrate that, in contrast to the reductase activity of its heme-free form, the green heme form of the protein reacts with hydroperoxides to generate highly unstable peroxide complexes.