Somatropin is a protein hormone that contains the same amino acid sequence as the human growth hormone produced by the pituitary gland. A robust and precise physicochemical chromatographic procedure is used in the assay to assign potency on a mass basis. Bioidentity is still required, and two procedure options are presented here: an in vivo bioassay procedure based on somatropin-induced weight gain in rats and a more precise rat cell line-based approach that measures production of ATP as a direct indicator of cell growth.