We have studied the efficacy of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) in stimulating haematopoiesis of patients with chemotherapy-induced myelosuppression. Ten patients with various myeloid and lymphoid neoplasias were treated daily with a single subcutaneous dose of rhGM-CSF (5 micrograms/kg/day), for a period of 5-10 days, after receiving highly myelotoxic chemotherapy. The treatment increased the white blood cell count (WBC) in nine of ten patients, primarily because of an increase in the number of neutrophils. Increase in bone marrow myeloid precursor cells, and myeloid to erythroid cell rations accompanied the white-cell response. In spite of this, five patients demonstrated rapid platelet recoveries, and in two patients erythrocyte levels increased after GM-CSF treatment. No toxicity was encountered with the cytokine therapy. Although rhGM-CSF was shown to stimulate haematopoiesis in patients with chemotherapy-induced myelosuppression, additional studies are needed to assess whether the use of GM-CSF will reduce chemotherapy-associated morbidity and improve response rates and survival among patients with neoplasias.