It is established that M protein is the cellular component of group A streptococci responsible for the production of type-specific antibodies, which are the basis of type-specific immunity in man. Strains freshly isolated from acute infections, so far as is known at present, always contain M antigen.

Although many cultures containing M protein are avirulent for mice, all highly mouse-virulent group A strains contain M antigen. The amount of M protein in some cultures can be increased by serial mouse passage, and the virulence of such strains is usually increased at the same time. It is possible that the hyaluronic acid capsule in addition to M protein may be a factor in virulence, but its importance has not been clearly defined. M protein probably enhances virulence by impeding phagocytosis, and the capsule may contribute to virulence by a mechanism not yet fully elucidated.

So far no completely purified preparations of M protein have been obtained, but progress in this direction is being made. Once this is achieved, it may be possible to define the properties of this important antigen more accurately.