The biological activity of the enterotoxin of Shigella dysenteriae 1 was compared with that of a well-studied 20-year-old partially purified preparation of neurotoxin from the same organism. Enterotoxicity, neurotoxicity, and cytotoxicity were present to an equivalent extent in both preparations. Human convalescent antisera and experimental rabbit antisera had equal toxin-neutralizing antibody to the cytotoxic activity in these toxin preparations. Multiple protein bands were present in each toxin studied. Two separate HeLa cell fractions could be obtained by Sephadex gel filtration chromatography, isoelectric focusing in a sucrose gradient, and polyacrylamide gel electrophoresis. Only one of these fractions (isoelectric at pH 7.2) was associated with enterotoxicity and neurotoxicity. The second smaller-molecular-weight fraction, which was isoelectric at pH 6.1, possessed only cytotoxic activity. These data suggest that Shiga enterotoxin and neurotoxin are closely related proteins and, indeed, may be identical. The nature of the cytotoxin with pH 6.1 is not clear, although it may be a subunit of the larger toxin that is capable of acting directly on the HeLa cell.