The effect of PAS against cancer and its immunoregulatory function were studied. The results indicated that PAS was able to inhibit tumor growth and prolong the survival time of tumor-bearing mice significantly at the dose of 100 mg/kg/day, ip. In addition, PAS was also able to increase the response of murine spleen cells to Con A and LPS, the amount of IgM and IgG PFC in murine spleen and the DTH response induced by BSA. These results indicate that the inhibitory effect of PAS on tumor growth is related to the enhanced immune response.