Both peptide elongation factors were purified from Zajdela and Walker tumors and from the host and normal liver. The activity of peptide elongation factor 1 from tumor tissues in promoting the binding of phenylalanyl-tRNA to ribosomes was significantly higher than that of normal liver. Also preparations from the host liver were markedly more active when compared with the corresponding factor from normal liver. Poly(U)-dependent phenylalanine polymerization was enhanced in subcellular systems containing elongation factor 1 from tumors or host liver. No differences were found between preparations of elongation factor 2 isolated from various sources. Tumor ribosomes showed an increased activity of both the acceptor and donor binding site of peptidyl transferase. In ribosomes from the tumor-host liver the activity of the acceptor site of this enzyme was decreased while that of the donor site remained unaltered. The enhanced activity of peptide elongation factor 1 from tumors and host liver is apparently the main reason of enhanced protein synthesis in these tissues. The enhanced activity of this factor is not specific for tumor growth as it occurs also in other pathological conditions.