IN a recent paper1 we showed that enterotoxin B isolated from Staphylococcus aureus strain S6 could be separated into two main protein fractions by starch-gel electrophoresis. After the completion of this work we received communications concerning this observation from Dr. M. S. Bergdoll (University of Chicago) and from Dr. E. J. Schantz (U.S. Army Laboratories, Fort Detrick). Dr. Bergdoll told us that he has demonstrated the presence of at least 3 toxic fractions of enterotoxin B which would react with the same antibody2 and suggested that denaturation during purification of the enterotoxin may be responsible for these fractions. Dr. Schantz informed us that he has obtained the two main protein components by starch-gel electrophoresis of purified enterotoxin. However, he was unable to resolve these by ultracentrifugation or by free electrophoresis; amino-acid analysis results were also consistent with .a single molecular species of one polypeptide chain3. Dr. Schantz suggests, therefore, that the starch-gel fractions are artefacts produced in the gel by the current and possibly by the reaction of impurities with functional (charged) groups on the protein. Accordingly we have tried to resolve our fractions by methods other than starch-gel electrophoresis and we have re-examined the purification procedure to find whether the fractions appear as artefacts as a result of any stage of purification.