Interferon alfa-2b (Intron A; Schering-Plough) was administered to 36 patients with chronic myeloid leukemia (CML) at an initial dose of 4 X 10(6) IU/m2 daily subcutaneously, adapted to changes in leukocyte counts during the course of treatment. Of 32 patients who could be fully evaluated (20 men and 12 women; median age, 34 years) 29 were in the chronic phase, one had a blast crisis and two had accelerated phase disease. Hematologic remission was achieved in 20 of the 32 patients, while a partial hematologic remission was obtained in 10. Elevated pretreatment white-cell counts returned to normal in 25 patients after 3-40 weeks. There was a parallel decrease in platelet counts after an average treatment time of six weeks and in lactate dehydrogenase, after 2-20 weeks. In conclusion, administration of interferon alfa-2b resulted in a relatively rapid cell reduction in chronic phase CML. The long-term effect of this treatment on the course of the disease and the place of interferon alfa-2b in the overall concept of CML treatment remains to be evaluated.