Two purified fractions of M protein of type 12 streptococci were further characterized. Only one of the fractions, “b,” continued to meet additional physical, chemical and immunologic requirements for homogeneity. The protein in this fraction had a molecular weight of 32,000, contained N-terminal alanine and C-terminal leucine, stimulated formation of bactericidal antibodies and did not contain the nontype-specific determinants found in the “a” fraction. The differences in molecular weights and antigenic complexity disclosed by comparisons of the proteins of the “a” and “b” fractions suggested certain structural relationships among the molecules of M protein of type 12 streptococci.

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