This epidemiological investigation revealed raw milk to be a source of mycobacteria, many of which resembled strains pathogenic for man and other animals. Eighty-three mycobacterial strains were isolated from 77 of 123 raw milk samples. The resemblance of these mycobacteria to known mycobacterial species and groups was as follows: 45, Mycobacterium phlei; 14, M. fortuitum; 1, M. smegmatis; and 23, slow growers (14 scotochromogens and 9 nonchromogens). Certain of the nonchromogenic slow growers may prove to be M. avium or M. bovis upon further bacteriological scrutiny.