The molecular basis of recombinant DNA technology is described, and the principles of genetically engineered proteins developed. The production of hGH by such methods utilizes a strain of Escherichia coli as host and a vector plasmid containing the appropriate information. Fermentation and purification of the hGH produced gives a preparation of high purity, containing only 1-2 ppm of E. coli polypeptide (ECP). This somatrem (Somatonorm) is identical to pituitary hGH except for an additional methionine residue at the N-terminal. Monoclonal antibodies fail to distinguish between pituitary hGH and somatrem. Preclinical studies of a variety of pharmacological and toxicological parameters indicate that the two hGH preparations have identical biological effects; no toxicological or mutagenic effects of somatrem have been detected.