The aim of the present study was to examine the relation between the grade of difficulty in cultivation of mycobateria and their development of cytochrome system as well as the difference of cytochrome system between the same strain of mycobacteria grown in vitro and in vivo. For this purpose, spectrophotometric investigations were made on cell suspensions or particulate fractions of four species of mycobacteria grown in vitro; M, smegmatis, BCG, M, tuberculosis avium, Flamingo strain and M, paratuberculosis and two species grown in vivo; BCG in vivo and M, lepraemurium.BCG cells grown in vivo were isolated from the hard nodule at the great omentum region of guinea pig, thumb size, formed due to the growth of the organism which had been injected intraperitoneally. The method of isolation consists of protease digestion of tissue fragments and centrifugal fractionation of bacterial cells. The viable cell number per unit weight of the cells grown in vivo was found to be approximately the same as that of BCG cells grown in vitro. The cells of M, lepraemurium, Hawaiian strain, were obtained from subcutaneous leprous nodules formed in mice, the isolation of the cells were carried out as described previously elsewhere.The results obtained indicated the presence in M, smegmatis of cytochromes of the a, b1 and c types, as well as a CO-binding pigment similar to cytochrome o, in BCG and M, tuberculosis avium, Flamingo strain, of cytochromes of the a, b and c types and in M, paratuberculosis of cytochromes of the a, b1 and c types. While no clear interrelation were evident between the cytochrome patterns or their cytochrome contents of these organisms and the ease with which they can be cultivated in vitro, the cells of BCG grown in vivo and these of M, lepraemurium were found to be completely deficient in cytochrome pigments.From the point of this similarity in the respiratory system of BCG grown in vivo and murine leprosy bacillus, that is the defect of cytochrome, it was presumed that further similarities in their respiratory mechanism might exist among these mycobacteria grown in vivo. Accordingly, it seems to be quite useful to study the respiratory system of BCG grown in vivo, which could be independent on cytochrome system, in order to contribute to the study on the metabolism of murine leprosy bacillus.