A streptococcal preparation, OK-432, was employed as the adjuvant immunotherapeutic agent combined with chemotherapy in cases of lung cancer. To evaluate the clinical efficacy of OK-432, patients admitted between 1975 and 1982 were randomized into two groups: an immunochemotherapy, or immunotherapy, group and a chemotherapy, or control, group. For each case, a fixed protocol of chemotherapy was administered using a combination of three drugs. In this study, cases with small cell lung cancer were excluded, and the survival rates of cases with non-small cell lung cancer were evaluated. One hundred fifty-nine cases in the immunotherapy group and 152 cases in the control group were eligible for evaluation of long-term survival rates. When comparing the prognosis of the two groups, statistically significant improvements of the survival rate in the immunotherapy group were noted in the following categories: all cases, resected cases, nonresected cases, resected stage I + II cases, resected stage III + IV cases, completely resected cases, incompletely resected cases, and cases with epidermoid carcinoma. From the results of the present study, it is concluded that OK-432 is a potent immunopotentiative agent in the treatment of lung cancer.