Abstract: The application discloses (A) a growth-promoting polypeptide having molecular weight from about 5,000 to about 7,000 chromatographic mobility under electrophoresis from about 0.25 to about 0.37 relative to lysine at pH 5 and from about 0.17 to about 0.43 relative to aspartic acid at pH 7.5, terminal amino acid at its amino end being asparagine or aspartic acid, and respective amounts of aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine, leucine, tyrosine, phenylalanine, lysine, arginine, trypophan, and cysteine or carboxymethyl cysteine, and also a couple of instances also methionine, isoleucine and histidine; and (B) the derivation of the polypeptides from blood serum or plasma or a plasma fraction by combined steps of (a) homogenizing the starting material in water, (b) admixing a solution of hydrochloric acid in ethanol thereby splitting the polypeptide from its carrier proteins and precipitating them, (c) adjusting the pH to 8.