Recombinant interferon alfa-2a (Roferon-A, Hoffmann-La Roche Inc., Nutley, NJ) has been evaluated in clinical trials of more than 1300 patients with a broad spectrum of oncologic disease. Patients with either solid tumors or hematologic malignancies were treated with daily or three-times-weekly intramuscular injections for induction periods ranging from 8 to 16 weeks. Doses ranged from 1 X 10(6) units to 124 X 10(6) units per injection. When administered in low daily doses (approximately 3 X 10(6) units), Roferon-A was well tolerated, and dose attenuation was rarely required. Change to three-times-weekly treatment regimen at the same dose was usually sufficient to control toxicity when it occurred in this group of low-dose patients. Those patients receiving higher doses frequently required dose attenuation to 50% of the starting dose to improve clinical tolerance. Virtually all patients treated with Roferon-A experienced some degree of acute toxicity manifested as fever, chills, myalgia, and/or headache. These reactions usually occurred with initial dosing and frequently improved spontaneously with continued administration of the drug. Acetaminophen pretreatment was generally useful in ameliorating these symptoms. Common adverse experiences occurring after repeated dosing included fatigue, anorexia, and weight loss. Serious adverse reactions including cardiovascular and neurologic toxicity have occurred infrequently, primarily at higher doses. Hematologic toxicity and elevations in liver function parameters were also observed, but rarely required dose attenuation. Adverse effects were usually reversible after dose reduction or discontinuation of therapy. Approximately 27% of all patients developed antibodies to rHuIFN-alpha 2A during treatment. No adverse clinical sequelae have been associated with antibody development to date.