Summary: A method is described for studying the postphagocytic fate of a bacterial surface antigen during degradation of the organism by macrophages. Observations of Group A, Type 1 streptococci after phagocytosis by unstimulated mouse peritoneal macrophages demonstrate cessation of bacterial growth, early partial loss of an immunofluorescent complex from the bacterial surface and later extrabacterial pooling of part of the complex in macrophage vacuoles. Evidence is presented which suggests that the observed changes in fluorescence reflect the fate of M protein still bound to its fluorescent label. The observations indicate, therefore, that M protein is removed from the bacterial cell wall and pooled within the surrounding macrophage vacuole. The significance of a mechanism within the macrophage which modifies particulate antigen is discussed in relation to initiating antibody synthesis and explaining the action of agents which alter the immune response.