SPG, a beta-1, 3 glucan extracted from cultured Schizophyllum commune Fries, was clinically evaluated for its efficacy in adjuvant immunochemotherapy for postoperative gastric cancer by the SPG cooperative study group comprising 43 hospitals. On the day of operation and the next day, patients were given intravenously 0.4 mg and 0.2 mg/kg of mitomycin C, respectively. Subsequently, they were randomly allocated to either the SPG group or control group. From the 10th to 20th postoperative days, the SPG group was intramuscularly given SPG at a dose of 20 mg twice a week or 40 mg once a week in combination with tegafur, while the control group was given only tegafur. Significant prolongation of life span of the SPG group was confirmed in Stage III. Minor side effects due to SPG were noted in 2.6% (5/190).