The heterogeneity of a pepsin extract of type-24 M protein (pep M24) was demonstrated by absorption of type-specific and cross-reactive human antisera with M protein fragments and heterologous serotypes of M proteins, pepsin extract of type-5 M protein (pep M5) and pepsin extract of type-6 M protein (pep M6). 2 of 12 individuals immunized with pep M24 developed significant rises in antibody titers against pep M5 and pep M6, as measured by the enzyme-linked immunosorbent assay. The sam individuals also developed opsonic antibodies against type-6, but not type-5, streptococci, which suggested the development of cross-protective immunity. Inhibition studies of one of these sera with the heterologous pep M proteins showed that the cross-reactive antibodies against pep M6 could not be blocked by high concentrations of pep M24, the immunizing antigen; these antibodies could be blocked, however, by cyanogen bromide-derived peptide fragments of pep M24, which suggested that the cross-reactive antibody was raised against an inaccessible site(s) in the pep M24 molecule. Inhibition studies of type-specific immune sera with pep M24 and peptides derived therefrom indicated that the M protein molecule contained multiple distinct as well as identical type-specific antigenic determinants that are unequally distributed among the seven derived peptide fragments.

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