Seventy-nine patients with small-cell lung cancer were treated with vincristin, methotrexate, and cyclophosphamide in inductive therapy and with methotrexate, cyclophosphamide, and procarbazine in maintenance therapy. Patients were divided at random into two groups: one group received chemotherapy alone and the second group was additionally subjected to systemic immunotherapy with Propionibacterium granulosum strain KP-45. In general, differences in the frequency of therapy response and in duration of remission could not be stated between the two groups of patients, but patients responding to chemotherapy showed a significantly longer remission time and lower complication rates. This benificial effect of chemoimmunotherapy is not related to a direct antitumor activity of the immunomodifier used, but to the lowered risk of myelosuppression and infections. Immunomodulation in combination with chemo- and/or radiotherapy can be recommended for the treatment of small-cell lung cancer.