1. Polytuftsin (Thr-Lys-Pro-Arg)n, was synthesized through polycondensation of an amino-free and carboxyl-activated derivative of tuftsin, H2N-Thr-Lys(Z)-Pro-Arg(Tos)-OSu, following suitable deprotection and fractionation steps. 2. Digestion of polytuftsin by trypsin, as well as by normal human serum, at 37 degrees C, yielded free tuftsin. 3. Polytuftsin affected the decreased formation of lung-metastasis, in B16 melanoma treated mice and prolonged the survival of animals more efficiently than tuftsin. 4. Tuftsin was found to be totally degraded by serum enzymes within approximately 60 min at 37 degrees C.