Effects of lysozyme (LY) on immuno-responses were examined. Tumor-bearing mice were used on most of experiments and normal mice on several cases of experiments. Cytostatic activity on the Peyer's patch cells in Meth-A bearing Balb/c mice was enhanced at 6 to 12 hrs after oral administration of LY. Furthermore, LY showed to recover from the decline of blastogenic activity, which was observed on the lymph node cells and the spleen cells of tumor-bearing mice, at 8 and 14 days after tumor inoculation. In the normal ICR mice, it was not observed that LY increased the foot-pad reaction against the sheep red blood cells (SRBC). On the other hand, the decrease of the foot-pad reaction, which was observed in the tumor bearing mice at 20 days after Sarcoma-180 (S-180) inoculation, was recovered by the administration of LY. A decrease of the number of plaque-forming cells (PFC) against SRBC which was induced with tumor inoculation in the spleen cells was not observed at 20 days after S-180 inoculation, but LY caused a significant increase of PFC response in spleens on both of normal and tumor bearing mice. From these results, it was suggested that potentiation of systemic immuno-response derived from the activation of Peyer's patches brought about antitumor effects of LY.