Marrow trepan biopsy was performed in 11 patients upon diagnosis of chronic myeloid leukemia and in clinicohematological remission of the disease. The patients were treated with alpha 2-interferon (reaferon) versus conventional myelosan (6 and 5 patients, respectively). The biopsies were studied histomorphometrically. In reaferon group the number of megakaryocytes and endosteal stromal cells diminished. This may be due to an indirect action of reaferon on hemopoietic tissue via endosteal cells of the marrow stroma. Reaferon is recommended for myeloid leukemia chronic with a granulocytic-megakaryocytic histological variant of the disease.