Bleomycin, a mixture of antibiotic polypeptides, was evaluated for therapeutic activity and clinical toxicology in 274 patients with far-advanced, nonresectable neoplastic disease. Therapeutic effect was most marked in advanced Hodgkin's disease, in which 50% of patients had significant objective and subjective improvement, in some cases for periods now approaching 2 years. Scattered responses of brief duration were seen in other neoplastic diseases. The clinical toxicity of bleomycin appears to be unique among antitumor agents; it produces no important effects on the blood-forming organs, gastrointestinal tract, liver, kidneys, or central nervous system. Pulmonary functional impairment, however, is common; irreversible pulmonary fibrosis, although rare, is a serious, sometimes lethal, manifestation of bleomycin toxicity that may limit its use in early neoplastic disease.