Twenty-three serotypes of atypical mycobacteria were investigated with regard to their ability to absorb the homologous agglutinins. The great majority of the strains of these types eliminated the homologous agglutinins revealing their antigenic identity with the standard type. Certain cross reactions between strains of 1 type and antisera of a 2nd type were analyzed, and criteria for their differentiation from the type-specific agglutination described.