Fifty-one patients with advanced lung cancer were divided at random into two groups. One group of 29 patients received the methanol extraction residue (MER) of Bacille-Calmette-Guerin (BCG) in addition to radiotherapy and/or chemotherapy. The other group of 22 patients was treated by radiotherapy and/or chemotherapy alone. The immune status of the patients was evaluated by skin tests to three recall antigens and KLH and by in vitro lymphocyte stimulation to PHA and three recall antigens. The immune status of both groups was similar before and after conventional treatment. The cutaneous reactivity, as well as the in vitro lymphocyte reactivity, became stronger in the MER-treated group, as compared with the control group. The MER-treated patients had less distant metastases. In a comparable clinical stage the survival in the MER treated group was slightly but not significantly better. This study has shown that MER stimulates the immune response, can be well tolerated and has few side effects. Therefore, MER treatment of patients with minimal tumor load can ethically be done with hope of obtaining better results.