A 23-year-old male patient with Philadelphia chromosome (Ph) positive chronic myelogenous leukemia (CML) was treated with both IFN alpha and IFN gamma. Normalization of leukocyte counts was reached after 3 months of treatment. Southern blot analysis failed to detect the neoplastic cell clone after 19 months of therapy. Cytogenetically, complete suppression of Ph positive cells in the patient's bone marrow and blood was observed after 20 months and 25 months, respectively. This response was achieved with doses of IFN alpha and IFN gamma which were considerably lower than the dosage of IFN used in single agent therapy of CML.