Contributions to the study of Salmonella. II. Protein determinants shared by S. typhi, S. enteritidis and S. typhimurium.

The antigenic relationship between and , that at present are known to share only the polysaccharide factors 9 and 12, has been investigated with a view to confirming the existence of common protein O specificity. A serum to absorbed with the polysaccharide of O-901 and a serum to in which the polysaccharide antibodies were present in extremely small amounts (undetectable in agar diffusion tests) precipitated with extracts containing proteins of . Additional evidence of the existence of common protein O factors was obtained by precipitin tests in agar in which sera to proteins from reacted with proteins of and (whose H determinants differ from one another); the precipitations of heterologous proteins in agar-gel varied in intensity according to the antibody content of the antiprotein sera tested and to the proportion of O determinants of the respective protein. Heterologous precipitin reactions of the sera to with antigens from were stronger than those with the homologous antigen, whereas the antigens from were but weakly precipitated by the sera to . This appears to indicate an intrinsically higher antigenic potency of , compared with .