Type-specific proteins of C. diphtheriae in the presence of heat may be converted into group-specific antigens. This indicates that the protein may not be a single antigen. To clarify this point rabbits were immunized with the proteins adsorbed to charcoal. Immune sera produced with 3 serological type-specific proteins gave, in general, specific agglutination and precipitation reactions, but when the more sensitive complement-fixation method was applied the sera were found to be less specific, marked cross reactions among the various proteins and sera being present. Evidently diphtheria protein contains, in addition to a common group antigen, at least one type-specific antigen which may be lost either by heating or by attenuating the organism.