Although somatotropins were discovered many years ago, only recent advances in recombinant DNA technology have made their mass production economically feasible. Somatotropins are growth hormones which occur naturally in animals. Their supplemental use in livestock production have been shown to increase animal productivity. For example, porcine somatotropin (PST) has the effect of improving the feed efficiency and carcass composition of pigs. Upon approval by the Food and Drug Administration, PST will become available to hog producers in the United States within the next few years.