Group A streptococcal M protein is known to be antiphagocytic; however, the exact basis for this property has not been established. In this study the hypothesis was tested that cell wall-associated M protein inhibits phagocytosis by interfering with bacterial opsonization. Two strains of group A Streptococcus pyogenes, CS44 (M+) and CS64 (an M variant of CS44), were radiolabeled, and after incubation in serum these organisms were exposed to human polymorphonuclear leukocytes. Phagocytosis was quantitated by measurement of leukocyte-associated radioactivity. The contributions of complement and of immunoglobulin to streptococcal opsonization were evaluated by use of serum from a variety of sources. The results revealed that the M strain was efficiently opsonized via the alternative complement pathway in a relative absence of immunoglobulins. In contrast, the M+ strain was poorly opsonized by all sera tested. These findings suggest that streptococcal M protein in some way prevents bacterial opsonization via the alternative complement pathway and that this property of M protein may partly explain its antiphagocytic characteristic.