Eleven patients with malignant tumors were given injections of potent interferon prepared from human leukocyte cultures. The preparation was given intramuscularly (since an intravenous injection to 1 patient was toxic) as 1–3×106 U of interferon 3 times weekly. Two patients had transient local reactions. All patients had increased sedimentation rates, a temperature rise dependent on interferon dose, and immunization against blood-group antigens, chick antigens, and parainfluenza type-1 virus antigens. No serious toxic effects were observed during treatment (150 days in 3 patients). Anti-interferon antibodies could not be demonstrated. No patient had clinical signs of acute viral disease during treatment. All but 1 patient tested had a drop in complement-fixing titers against an array of viruses and mycoplasma and stable or decreasing hemagglutination-inhibiting titers against rubella virus antigen. Three patients with disseminated herpes zoster had relief of pain and developed crust formation within 1 week after initiation of interferon therapy. No definite general antitumor effect was observed in this small group of patients during interferon therapy. The study strongly suggests that high-dose, long-term interferon administration is possible in man.