SUMMARY. The pathogenicity of 40 strains of Mycobacterium avium, M . paratuberculosis, M . intracellulare and M . lepraemurium was * investigated in chickens, rabbits, guinea-pigs, mice and calves. Mycobactin dependence and serological type were also determined. There was no evidence that mycobactin dependence was related to pathogenicity. Antigenic similarities were demonstrated between M . avium and M . paratuberculosis, and one isolate had the pathogenic characteristics of both species.