Immune responses have been reported to decline following surgical stress, leading to an increased susceptibility to infection or to the growth of tumors. In this study, we report that pre-operative treatment with the biological response modifier (BRM) polysaccharide Kureha (PSK) can inhibit the decline of immunocompetence during the perioperative period. BALB/c mice were laparotomized, the intestinal tracts were exposed to room air for 1.5 h, and then the abdomens were closed. Six hours after the operation, the spleen and serum were collected. The concentrations of IL-6 in the serum and of IFNgamma and IL-4 in the supernatant of splenocyte cultures were measured. In the surgical stress group that received surgical stress and the PSK treatment, the serum IL-6 concentration was significantly elevated in the group with surgical stress. PSK treatment controlled the stress-induced elevation. IFNgamma concentrations were measured in the supernatant of Concanavalin A (Con A) stimulated splenocyte cultures. It was lower in the group with surgical stress than in the cultures derived from the non-treated group. The IFNgamma concentration in the group with surgical stress plus PSK treatment was significantly higher than the level in the group with surgical stress alone. The IL-4 concentration was significantly lower in the surgical stress group than in the control group, however, the concentration tended to be higher in the surgical stress plus PSK treatment group than in the group with surgical stress alone. The IFNgamma/IL-4 ratio in the group with surgical stress was lower than the ratio in the non-treated group. The ratio in the group with surgical stress plus PSK treatment was significantly higher than the ratio in the group with surgical stress alone. These results suggest that PSK restores the abnormality of the biological responses induced by surgical stress and corrects the reduced Th1/Th2 cytokine balance to a normal level.