Recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) was administered to 10 patients with refractory malignancies, 2 patients who had myelodysplastic syndromes with severe neutropenia and to a patient who had delayed marrow recovery after 3 cycles of therapy for acute leukemia. A marked neutropenia and monocytopenia was observed within 5 min after an i.v. injection of GM-CSF. This persisted for 1-2 h and seemed related to activation of an adhesive glycoprotein (MO1) on the surface of these cells. With continued daily i.v. administration of GM-CSF, all patients with refractory malignancies developed a striking leukocytosis. Total leukocyte counts reached 75,000/microliters within 2 weeks of treatment. This was due to an increase in band and segmented neutrophils, eosinophils and monocytes. Accelerated myelopoiesis required the continuous presence of GM-CSF; with pump failure for 24 h or discontinuation after 14 days, leukocyte counts returned to normal levels in 24-48 h. GM-CSF also increased myelopoiesis in the patients with myelodysplastic syndromes or following anti-leukemic treatment. These observations suggest that this growth factor should prove a useful adjunct in the treatment of patients with malignancies and bone marrow failure.