CYTOCHROME a2, which is found in certain bacteria, for example, Aerobacter aerogenes and Azotobacter, has particular interest as it may play the part of a cytochrome oxidase in these organisms. The main absorption band of this cytochrome in the visible part of the spectrum is in the red region : reduced, 630 mµ; oxidized, 645 mµ; carbon monoxide complex, 635 mµ. Spectroscopic similarities to some ferrophæophorbides1 as well as to certain bile pigment iron complexes2 have been observed.