Purified M proteins were recovered from acid extracts of , M-types 1, 3, 6, 12 and 17, by elution from columns of hydroxyapatite of the proteins precipitated with ammonium sulphate. M protein free from non-type-specific antigens was recovered only from M-type 12. Although similar fractions were not recovered from M-types 1, 3, 6 and 17, purified preparations containing a single cross-reactive antigen were obtained. In addition to the M proteins associated with cross-reactive antigens, type-specific antigens that did not stimulate opsonic antibodies were isolated from the acid extracts of M-types 3 and 17. Further study of the purified M proteins revealed molecular weights that ranged from 32 000 to 63 000 daltons, total amino acid compositions that were similar, and N-terminal amino acids that were variable.


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