Amino acids are incorporated at increased rates into hepatic proteins in tumor-bearing humans and animals. In this study, we hoped to elucidate whether this is an expression of increased hepatic protein synthesis or altered isotope dilution in the precursor pool(s). Liver tissue from sarcoma-bearing mice (MCG 101) showed increased specific activities of arginine and leucine in hepatic proteins after i.p. injection of these precursors. The specific radioactivity of leucine in the free amino acid incorporation rate into proteins was also found in incubated liver slices and in a cell-free system of incubated free and membrane-attached polysomes. The increased amino acid incorporation was the net result of increased as well as decreased relative turnover of various hepatic proteins. The hepatic content of RNA was increased, but hepatic DNA and protein content was unchanged in tumor-influenced livers. Increased amino acid incorporation into hepatic proteins in tumor-bearing animals and also probably in cancer patients is due to a net increased hepatic protein synthesis, probably not confined to acute-phase reactants only.