Macrophage colony-stimulating factor (M-CSF) is a homodimeric glycoprotein which stimulates differentiation of progenitor cells to mature monocytes and enhances production of hemopoietic growth factors from mature monocytes such as granulocyte-macrophage CSF, granulocyte CSF and megakaryocyte potentiator, suggesting that M-CSF administration enhances production of monocytes, neutrophils and platelets. Since the commercial availability of M-CSF in 1991, serial M-CSF infusions at a daily dose of 8 million units have been performed in patients with acute myeloid leukemia in complete remission after combination chemotherapy. Although M-CSF infusion reduced the duration of neutropenia in 3 of 5 patients, it reduced the duration of pyrexia over 37°C in all patients, and that over 38°C in 4 of 5 patients. Average durations of pyrexia and parenteral antibiotic injections were significantly shorter in M-CSF than in control courses. Although M-CSF infusion reduced the duration of thrombopenia in 2 of 5 patients, it reduced total platelet units transfused in 4 of 5 patients. The average number of platelet units transfused after combination chemotherapy was significantly lower in M-CSF than in control courses. In mice, M-CSF injection increased the serum concentration of reactive nitrogen intermediates which inhibited the growth of L1210 cells, and increased the survival rate of mice previously injected with them. These results indicate that M-CSF may be a promising agent not for improving patients’ quality of life after cancer chemotherapy, but also in cancer immunother