Mycobacterium avium infects a number of animal species and have become the most common disseminated bacterial infection in AIDS patients. The bacillus is also found in the environment. Using PCR specific to IS901, and ELISA based on monoclonal antibodies specific for p40 as well as serotyping, strains from different sources were examined for the distribution of these markers. IS901 and p40 were found concomitantly in all of the examined strains. These two markers were found in "wood pigeon" isolates as well as in M. avium strains isolated from animals (found in most serotype 2 isolates and in some serotype 1 and serotype 3 isolates). The two markers were not found in M. paratuberculosis isolates. Approximately 50% of the isolates from animals contained the two markers. The markers were not identified in strains isolated from the environment. The distribution of serotypes and markers differed in human and animal isolates, suggesting that animals are not the source of human infection. Marker positive and marker negative strains were isolated from animals with approximately equal frequency and the strains could not be distinguished by cultivation or by the pathological changes caused by the bacteria. This could indicate that the two markers are not directly related to virulence. Though the two markers seems to have substantial potential to differentiate between strains of the M. avium complex, the presence of the markers was found inconsistent with the present grouping of mycobacteria.