Studies are centered on the physicochemical and immunochemical analysis of the group A streptococcal M protein, a surface antigen of the organism. This protein is intimately associated with streptococcal virulence through its property of inhibiting phagocytosis of the organism. Although there are a large number of serologically distinct M proteins among the many types of group A streptococci, they share the common biological property of the antiphagocytic effect. At present, two serologically unrelated M proteins have been purified and partially sequenced. Physiocochemical and sequence data indicate that the M molecules have a close structure relationship with mammalian tropomysin.