Electron microscope studies demonstrated that soluble proteins (ferritin and horseradish peroxidase) administered in vitro to strips of guinea pig diaphragmatic muscle under physiological conditions were internalized by vesicles of primitive reticular cells (PRCs) found in the connective tissue. This endocytosis effect was enhanced when the muscle preparations were obtained from animals actively allergized to the protein. Denervation increased the occurrence of peroxidase-positive vesicles in the peroxidase-antiperoxidase system. These results suggest that the antigen-antibody interaction induces endocytosis directed to the specific antigen.