Mycobacterium kansasii has a strong virulence as compared with those of "mycobacteria other than tubercle bacilli". In this study, hemolytic activities of several strains of M. kansasii were investigated. Secretion of hemolytic factor from these strains was not observed, since no hemolytic zone was found around the colonies grown on the 7H11 agar plate when 5% rabbit blood agar was overlayed. On the other hand, weak hemolytic activities were detected after treatment of sonication and trypsin-digestion of M. kansasii cells. They were also heat-resistant. These results indicate that endogenous nonproteinaceous factor was involved in the hemolytic activity of M. kansasii. Moreover, these activities were not species-specific in respect to the kind of target erythrocytes used for the assay. However, hemolytic activity of M. kansasii was not increased in spite of the change of culture conditions or supplement of nutrients into the medium. It is unlikely that this hemolytic activity is directly associated with a pathogenesis of M. kansasii, but detailed studies were needed to clarify on this point.