Some biological characteristics of organisms composing the Mycobacterium avium-Mycobacterium intracellulare complex (MAC) are considered. Some subspecies of MAC organisms are characterized, and the most important risk factor for MAC infection in birds and mammals is revealed. The cultural-biochemical, serological, chemotaxonomic, and genotypic methods for the identification of MAC organisms are estimated. The wide distribution of MAC strains in the environment has been detected. Their new sources, such as water pipes, basins, washhouses, and others, have been found. The role of biofilms in the colonization of new surfaces by mycobacteria is presented, and their resistance to disinfectants and medications is revealed. A high pathogenicity of MAC organisms for immuno-compromised patients of various etiology is ascertained.