A pathologic event in humans or animals that involves the participation of two or more species of mycobacteria may be designated as a mixed mycobacterial infection. In the majority of cases of such mixed infection, Mycobacterium tuberculosis is found with one of the atypical mycobacteria. The allergic and immune reactions produced by Mycobacterium simiae, Mycobacterium asiaticum, and Mycobacterium chelonei in guinea pigs was studied. Sensitins that were prepared from cultures of these microorganisms were species specific, producing intensive cutaneous reactions in the animals infected with these organisms. Animals that received atypical mycobacteria and were infected secondarily with bacille Calmette-Guerin (BeG) had a positive reaction to tuberculin H37Rv PPD-S, but the sensitivity of such animals to the atypical strains was not altered. The atypical strains studied have no immunogenic power against tuberculosis, and they reduced the immunizing power of BCG, probably by exerting an immunosuppressive effect.