The immunomodulatory activity of PJ-P, a polysaccharide fraction extracted from Paeonia japonica, were reported in our previous paper. In the present study, we investigated that PJ-P inhibited cancer growth through activation of macrophages. The activities of peritoneal macrophage to induce tumor necrosis factor (TNF)-$alpha$, interleukin-1 (IL-1)$eta$, interleukin-6 (IL-6) and interleukin-12 (IL-12) as well as to ingest fluorescence-latex microbeads were enhanced by treatment of PJ-P. Direct cytocidal activity of PJ-P against cancer cells was not shown. However, in vitro, peritoneal macrophages treated with PJ-P had an activity to kill cancer cells. Furthermore, PJ-P significantly prolonged the survival of mice implanted intraperitoneally with B16F0 mel-anoma cells. These results suggest that PJ-P could be a useful immunomodulator and assistant of anti-tumor agent.