The carbohydrate of the cell wall of group C streptococci is one of the best known group-specific streptococcal antigens with respect to its chemical structure. In the present paper, the ultrastructural location of the carbohydrate was investigated by means of immunoelectronmicroscopic techniques. Besides group-specific antibodies, the specific binding of an agglutinin (protectin Anti-AHP) of the edible snail (Helix pomatia) to structures with terminal N-acetylgalactosamine was used to demonstrate the antigen. The agglutinin was extracted from the albumen gland of the snail and purified by column chromatography. By means of glutardialdehyde it was coupled with ferritin or horesradish peroxidase. The investigations were done on strains of Streptococcus equisimilis and Streptococcus equi. Str. pyogenes (group A streptococci) was used as a control. On whole cells of group C streptococci the carbohydrate was demonstrated on the surface of the triple-layered cell wall. Near the cross-wall the carbohydrate seems to be more concentrated. In the presence of N-acety-D-galactosamine the labelling of the cell wall was inhibited. N-acetyl-D-glucosamine did not have such an effect. On isolated cell walls, both the outer and the inner surface were tagged. This suggests that the group-specific carbohydrate covers the peptidoglycan (mucopeptide) on both sides. The cytoplasmic membrane shows no reaction.