McCord and Fridovich [ 1,2] have shown that erythrocuprein, the copper and zinc containing protein extracted from red blood cells, can enzymatically disproportionate superoxide radicals (Oh), an intermediate of the reduction of oxygen, the generation of which in some Ha 0s -producing biological oxidations has been demonstrated [3]. Khan has recently pointed out [4] that dismutation of Og gives rise to the production of the highly reactive singlet state of oxygen, which can be revealed by its own luminescence; indeed Arneson [5] observed that oxidation of xan’thine by xanthine oxidase produced luminescence, which was abolished by the presence of erythrocuprein. On the basis of these data, the main purpose of the following paper is to show that erythrocuprein has a quenching effect on luminescence phenomena which are reasonably ascribed to singlet oxygen produced even in the absence of superoxide intermediates. Thus the physiological role of erythrocuprein could be to catalyze the singlet-triplet conversion of the oxygen states: in other words to afford an enzyme-regulated dismutation liberating in solution as a product the inert ground state triplet oxygen, rather than simply accelerating the dismutation reaction.