metric investigations were made on cell suspensions or particulate fractions of four species of mycobacteria which had been cultivated in vitro. The results obtained indicated the presence in Mycobacterium smegmatis of cytochromes of the a, bi, and c types, as well as a CO-binding pigment similar to cytochrome o; in BCG and M. avium, cytochromes of the a, b, and c types were present; and, in M. paratuberculosis, cytochromes of the a, bi, and c types were present. Although no clear interrelations were evident between the cytochrome patterns of these organisms and the ease with which they can be cultivated in vitro, it was found that the total reduced nicotinamide adenine dinucleotide oxidase activity of the particulate fraction was remarkably low in M. paratuberculosis, which can be grown in vitro only with great difficulty. The cells of BCG grown in vivo and those of M. lepraemurium isolated from leprous nodules of mice were found to be completely deficient in cytochrome pigments.