Abstract
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.