Monkey growth hormone (MGH) has been purified by reverse-phase high-performance liquid chromatography (HPLC). Tryptic digests of MGH were separated by HPLC and paper electrophoresis. From amino acid composition, from NH2-terminal residue and sequence analyses of these tryptic peptides, and from their alignment with those of human growth hormone, the primary structure of MGH was proposed. There are only four residues which are different in these two growth hormone molecules.