Abstract: A cell growth inhibitor consisting essentially of a peptide substantially free of cytotoxins and containing an amino acid sequence of Phe-Cys-Arg-Phe-Leu-Leu-Cys-Pro-Ser-Arg-Thr-Ser-Asp. The peptide may be obtained by fractionation of venom from proteroglyphodont snakes. One cytostatically active fraction contains a peptide with a molecular weight of 3200-3500 daltons, 27-29 amino acid groups, and a blocked amino terminus. Another fraction contains an oligomer or precursor of the lower molecular weight peptide. Also disclosed is a method of inhibiting the growth of cells by introducing the cell growth inhibitor to the cells to be inhibited in an effective amount.