Streptococcal M protein has been studied directly in the intact streptococcal cell by specific immunofluorescence. By this method, it can be seen to be concentrated in or on the cell wall, but cannot be detected in the capsule. The lack of type-specific (but not group-specific) immunofluorescence after trypsinization; and the inhibition of group-specific immunofluorescence by unlabeled type-specific antibody, are observations most compatible with a location of the M antigen determinants on the cell surface superficial to the group antigen.

M antigen is not "resynthesized" after trypsinization of living cells, but appears anew only at sites of new cell wall growth. A limited amount of such growth, leading sometimes to detectable amounts of M in the gross, can take place in deficient media without detectable increases in optical density of the cell population.

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