Group A streptococcal M protein was extracted with nonionic detergent and subjected to a number of physical, chemical, and immunological tests. M protein thus extracted was composed of multiple protein bands, ranging from 35,000 down to 6,000 daltons, all having type-specific precipitating activity. The anti-phagocytic proteins, however, were limited to three molecular species having mol wt of 28,000, 31,000, and 35,000 daltons, and could be separated from those proteins that had only type specificity. Physical studies indicated that these proteins existed as individual asymmetrical molecules which were not aggregated. By radiolabeling M protein on living streptococci, it was determined that these protein bands were found on the streptococcal cell wall in this multiple form. Also, by pulse chase experiments supported by chemical and immunological data, evidence was obtained strongly suggesting that the smaller, type-specific molecules are used to assemble the larger, antiphagocytic proteins.