Slow-growing, non-pigmented mycobacteria from tuberculous swine and avian strains were found to closely resemble Group HI (Battey) organisms from human sources. The only difference in behavior among these groups of mycobacteria was that the avian and swine strains were virulent for fowl in small doses (0.1 mg) whereas Battey strains produced disease only in higher doses (1 mg). No difference was noted, however, in the histopathologic appearance of lesions produced by all three groups of acid-fast bacilli. The close antigenic relationships of swine, avian, and Battey strains were suggested by the more intense and similar response of sensitized guinea pigs to avian tuberculin than to human tuberculin. It is proposed, therefore, that Battey strains are variants of M. avium of lower virulence for chickens. Although swine were not definitely established in this study as a source of Battey infection, any animal species which harbors M. avium, in view of this organism''s long survival in soil and its high thermal resistance, may constitute a hazard to public health. Food, milk, and water contaminated by soil may well be the source of Battey infection.