A phase II study examining the effects of human recombinant granulocyte-colony-stimulating factor (G-CSF) on the growth of colony-forming unit-granulocyte macrophage (CFU-GM) in the bone marrow and in the peripheral blood was performed in lung cancer patients treated with cisplatin-containing regimens. Treatment with G-CSF following chemotherapy significantly increased the absolute granulocyte count. No significant effect of G-CSF on either the platelet or the red blood cell count was observed. Treatment with G-CSF did not affect the CFU-GM levels in the bone marrow, but did have a significant effect on peripheral blood CFU-GM levels 14 days after initiation of chemotherapy (P less than 0.05). Four patients demonstrated a rebound increase in the level of peripheral blood CFU-GM during the first course of chemotherapy without G-CSF. In contrast, eight patients displayed increase in peripheral blood CFU-GM levels during the second course of chemotherapy with G-CSF treatment. These findings demonstrate that G-CSF is a potent stimulator of granulocyte proliferation as well as a potent agent for promoting transport of hematopoietic progenitors from the bone marrow into the peripheral blood.