A neutral polysaccharide fraction (SMPA) prepared from the roots of Salvia miltiorrhiza by DEAE-cellulose and Sephadex G-100 chromatography was tested for its immune enhancing function in N-methyl-N'-nitro-nitrosoguanidine (MNNG)-induced gastric cancer rats by intragastric administration. SMPA (200mg/kg) treatment not only increased the body weight, but also improved the immune organ indices. Furthermore, studies of various immunological activities in gastric cancer rats revealed that SMPA significantly stimulated splenocyte proliferation, promoted anti-inflammatory cytokines (IL-2, IL-4 and IL-10) production, inhibited pro-inflammatory cytokine (IL-6 and TNF-α) secretion, augmented the killing activity of natural killer (NK) cells and cytotoxic T lymphocytes (CTL), and increased phagocytotic function of macrophages in gastric cancer rats. In addition, SMPA administration evidently elevated total intracellular granzyme-B and IFN-γ levels produced by splenocytes in gastric cancer rats. Taken together, these results suggested that SMPA could act as an effective immunomodulator and might be explored as a potential supplemental source for gastric cancer therapy.