The anti-tumor activity of a new type of peptidoglycan isolated from squid ink was shown to have a cure rate of 64% for Meth A tumor from BALB/c mice. The ink delipidated in acetone, which contained the peptidoglycan at 0.1% (w/w), was administered to tumor-transplanted mice so as to examine the anti-tumor activity. One-fifth of the tumor-bearing mice was cured with 3 injections (1 mg/head) of the acetone delipidated squid ink or a prolongation of survival was observed in the treated animals. Heat treatment at 100 degrees C for 10 min did not affect the anti-tumor activity of the delipidated ink, its potentiality being preserved. The acetone-extractable fraction of the ink also brought about a similar cure rate for Meth A tumor. The delipidated ink enhanced the phagocytic activity of macrophages but no direct cytotoxicity was observed for the Meth A tumor cells. Hence it may be said that the anti-tumor activity of the delipidated ink was mainly due to the augmented cellular immunity in vivo.