Human disease caused by organisms in the Mycobacterium avium complex occur virtually worldwide. A 20-year ongoing study conducted in Western Germany has been analyzed to elucidate the ecologic and epidemiologic characteristics of these infections in man. Organisms included in this investigation have been cultured from man, from domestic and wild animals and fowl, and from a variety of environmental sources. In addition to the usual taxonomic studies of these bacilli, infrasubspecific typing by seroagglutination has enabled identification of 3 distinct serogroups: the classical Mycobacterium avium strains (serovars avium 1, 2, and 3), the intermediate group (avium serovars 4, 5, 6, 8, 9, 10, and 11), and the less frequently encountered organisms, the 11 remaining avium serovars (7 and 12 through 21). Analysis of the number of strains in each of the 3 serogroups, derived, respectively, from man, from animals, and from the environment, has enabled us to draw some conclusions regarding reservoirs and sources of human infection with these agents.