Human leukocyte interferon, HuIFN-alpha (LE), has been tested in combination with radiotherapy and chemotherapy for previously untreated small cell lung cancer. Nine patients with limited disease received high-dose IFN followed by a low-dose regimen; and six patients had a low-dose regimen from the beginning. The high dosage of IFN consisted of 800 X 10(6) IU given as a continuous intravenous infusion for 5 days, followed by 6 X 10(6) IU i.m. three times weekly. If the first site of disease progression was local or in a central nervous system location, radiotherapy (55 Gy/20 F/7 weeks locally and/or 30 Gy/10 F/2 weeks whole brain) was applied and IFN was continued. Chemotherapy was administered only if there was disease dissemination outside the chest. Three patients achieved minor response for as long as 20, 25, and 42 weeks, respectively, with IFN alone. Three of five complete responders to IFN-radiotherapy died 18, 33, and 41 weeks from the start of IFN treatment without chemotherapy. Autopsy did not reveal macroscopic or microscopic tumor at any site, but there was severe radiation pneumonitis. Four of nine patients were administered chemotherapy subsequent to IFN-radiotherapy because of disease dissemination. The median length of survival of the entire group was 41 weeks. On the low-dose regimen, one patient achieved partial response with IFN alone (duration, 12 weeks); of five evaluable patients three achieved complete remission and two partial remission to IFN-radiotherapy, and one of the three complete responders to IFN-radiotherapy died of severe radiation pneumonitis at 21 weeks from the start of IFN treatment. No tumor was detected at autopsy. The study is in progress. Average survival at present is 33 weeks. The results derived from both our studies suggest a growth-delaying effect of HuIFN-alpha (Le) on small cell lung cancer. They also suggest potentiation of radiation by HuIFN-alpha (Le). Memory and psychomotor dysfunction, fatigue, and anorexia were dose limiting with both short-duration, high-dose and long-duration, low-dose IFN therapy. We feel that IFN, as part of a combined multimodality treatment of small cell lung cancer, may play a role by delaying metastatic dissemination.