A new intermediate of heme degradation was found in the heme oxygenase reaction with the heme•heme oxygenase complex and the NADPH-cytochrome c reductase system. The intermediate showed an absorption peak at 688 nm and its carbon monoxide complex gave a peak at 638 nm. The 688 nm substance seemed to have an extremely high affinity for carbon monoxide, and when the reaction was performed under a gas mixture of carbon monoxide and air, the reaction stopped almost completely at the level of the 638 nm substance. The same intermediate also appeared when the heme•heme oxygenase complex was incubated with ascorbic acid. The carbon monoxide complex of the 688 nm substance was easily and quantitatively converted to biliverdin-iron chelate when exposed to air. The heme moiety of the 688 nm substance was extracted in chloroform and compared with protoheme and verdohemochrome by thin layer chromatography. The sample migrated faster than either verdoheme or protoheme. The pyridine complex of the heme moiety of the 688 nm substance was not converted to pyridine verdohemochrome when incubated in air in the presence or absence of ascorbic acid, indicating that the 688 nm substance is not involved in the intermediary step of heme degradation in the coupled oxidation of pyridine-hemin. In other words, the sequence of heme degradation catalyzed by the heme oxygenase system may differ in some respects from that in the coupled oxidation of pyridine-hemin. In addition, biliverdiniron chelate failed to afford pyridine verdohemochrome on the addition of pyridine. The chemical nature of the new intermediate remains to be elucidated.