Fifty-two patients with advanced cancer received sequentially escalating doses of 3 to 50 million units of recombinant DNA-produced alpha interferon by daily intramuscular injection. There were 23 patients with metastatic breast cancer, 17 patients with nodular poorly differentiated lymphocytic lymphoma, and 12 patients with multiple myeloma. Complete and partial remissions were obtained in 35 percent of patients with nodular poorly differentiated lymphoma, whereas rare activity was found in breast cancer and multiple myeloma. Dose-limiting toxicity occurred in patients receiving 36 million units or more and consisted of fatigue/asthenia, weight loss, and elevation of transaminase levels, requiring frequent interruption, reduction in dose, or cessation of treatment. Hematologic toxicity was rarely a limiting factor, but myelosuppression was severe in some patients with multiple myeloma. All toxicities were reversible on discontinuation of treatment. Antibodies to recombinant leukocyte A interferon were seen infrequently but may adversely affect therapy.