In numerical classification, four species of the Mycobacterium nonchromogenicum complex, Mycobacterium nonchromogenicum, M. terrae, M. novum, and M. triviale, formed one cluster. These four species appeared to be reduced to one species, Mycobacterium nonchromogenicum. Furthermore, relationships between the species were numerically analyzed by using the hypothetical median organism pattern. The results showed that the M. nonchromogenicum complex can be divided into two subgroups: M. nonchromogenicum and the other three. These two subgroups were differentiated from each other by scores based on two or more positive reactions in the following three characteristics: resistance to bleomycin (5 pgjml); heat-stable acid phosphatase activity; nicotinamidase or pyrazinamidase activity or both activities. M. nonchromogenicum gave two or three positive reactions among these three, and M. terrae, M. novum, and M. triviale gave two or three negative reactions.Three cases of lung infection due to M. nonchromogenicum, as well as three other cases of probable lung infection due to M. nonchromogenicum, were observed in this study. Only one organism isolated from one doubtful case was M. terrae. Up to now, M. nonchromogenicum was considered a nonpathogen. It was shown, however, that this organism causes lung infection in humans. Wayne 1966 (36), M. novum Tsukamura 1966, and M. triviale Kubica et al 1970 (8) are closely related, slowly growing, nonphotochromogenic mycobacteria (16, 24). They form one cluster in numerical classification (24), and, based on this finding, were named the Mycobacterium nonchromogenicum complex (28). These organisms have been considered nonpathogenic, but recently there appeared several papers reporting infections due to these organisms (2,3,5,6,26). As will be stated below, the causative organisms were not adequately identified in these studies. The reason is the uncertain knowledge of the taxonomy of these organisms. There exists, at present, no reliable method of identification of these organsims. The 219
Mycobacterium nonchromogenicum Tsukamura 1965 (17), M. terrae