Preparations of cholera toxin prevented HeLa cells from spreading on a plastic surface. The toxic factor was heat labile. The toxin was filtered through Sephadex G25 and the cytotoxic component appeared to enter the resin, and cytotoxic fractions sensitized sheep cells to antitoxic serum. These fractions were then run on a G50 column and amino acid analysis suggested the presence in active fractions of a peptide containing 11 amino acids.