The sharing of one and possibly two or more non-type-specific antigens by most strains of groups A, C, and G streptococci is described. With the exception of a single strain of Proteus mirabilis, this antigen(s) was not found among strains of groups B and D streptococci, coagulase-positive staphylococci, Escherichia coli, Pseudomonas sp., and Salmonella sp. The non-type-specific antigen(s) could not be separated from M protein by fractionation with various saturations of ammonium sulfate or by column chromatography with calcium hydroxylapatite even though the latter method allowed the recovery of a fraction of M protein which was free of the cross-reactive antigen(s). The resistance of this non-type-specific antigen(s) to hot acid extraction and its sensitivity to treatment with trypsin differentiate it from the T and R antigens of group A streptococci, both of which are trypsin resistant. Co-precipitation of both type-and non-type-specific antigens occurred with type-specific antiserum and suggested that the type- and non-type-specific antigens represent either different, covalently bonded antigenic determinants on the same protein or different proteins noncovalently linked together.