A streptococcal preparation, OK-432 was employed as the adjuvant immunotherapeutic agent for lung cancer. To evaluate the clinical efficacy of the OK-432, patients admitted between 1975 and 1979 were randomized into two groups: (1) an immunochemotherapy group and (2) a chemotherapy, or control, group. For evaluation of long-term survival, there were 108 cases in the immunochemotherapy group and 103 cases in the chemotherapy group. When comparing the prognosis of the two groups, the survival rate was statistically higher in the immunochemotherapy group than the control group. The resected cases in Stages I and II showed better prognosis with immunochemotherapy than the cases in the same stages treated with chemotherapy alone. Among the resected cases in the more advanced stages, mostly Stage III and a few cases in the Stage IV, the cases treated with immunochemotherapy also showed more favorable prognosis than the cases treated with chemotherapy alone. In terms of the cell type of the lung cancer, the cases with epidermoid carcinoma in Stages I and II showed significantly better prognosis with immunotherapeutics than the control group. The cases with positive reaction to the streptococcal polysaccharide skin test apparently showed better prognosis than those with negative reaction.