Daily administration of recombinant porcine somatotropin (rpST) has marked effects on growth performance of swine. Protein accretion rates are dramatically increased with rpST treatment. However, the effect of rpST on specific muscle proteins has not been addressed. The purpose of this study was to examine the effects of rpST on 1) the distribution pattern of isomyosins, 2) the total myosin heavy chain protein levels, and 3) myosin heavy chain mRNA abundance present in swine skeletal muscle. There were no differences in myosin isoform composition in the longissimus muscle resulting from porcine somatotropin treatment. There were also no differences in the myosin heavy chain protein concentration within the longissimus muscles. In accordance with the protein levels, myosin heavy chain mRNA expression was not altered with rpST treatment. This study shows that although the pig responds to rpST with increased protein accretion rates, the proportion of myosin per milligram of total protein remains constant. Myosin was not altered by type, concentration, or expression level. By combining these tools we are now able to examine a myosin on both a protein and message level throughout growth.