1. 1. Polytuftsin-(-Thr-Lys-Pro-Arg-)-n, was synthesized through polycondensation of an aminofree and carboxyl-activated derivative of tuftsin, H2N-Thr-Lys(Z)-Pro-Arg(Tos)-OSu, following suitable deprotectoin and fractionation steps. 2. 2. Digestion of polytuftsin by trypsin, as well as by normal human serum, at 37°C, yielded free tuftsin. 3. 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. 4. Tuftsin was found to be totally degraded by serum enzymes within ~ 60 min at 37°C.