Exo-alpha-N-acetylgalactosaminidase has been purified 8000-fold from Clostridium perfringens by gel filtration, ion exchange chromatography, isoelectric precipitation, and negative adsorption on human O type erythrocytes. The resulting enzyme is active at physiological pH and temperature. Phenyl glycosides, oligosaccharides, mucins, glycolipids, and cell membranes are substrates for this enzyme. The result of enzyme action on blood type A erythrocytes is the loss of A activity and the simultaneous appearance of H activity, characteristic of the O blood group type. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate demonstrates electrophoresis in sodium dodecyl sulfate demonstrates that the blood group A-destroying activity is distinct from the other glycosidase activities found in C. perfringens.