The nature, type and mechanism of action of various colony stimulating factors (CSFs) have been described. Among these CSFs, injection of the recombinant human granulocyte CSF (rhG-CSF) caused a marked increase in neutrophils in mice as well as in monkeys. The neutrophilia of injected mice were preceded by a marked increase of hematopoietic precursors in hematopoietic organs. Injection of monkeys with rh granulocyte-macrophage CSF (rhGM-CSF) also induced a marked increase in peripheral blood neutrophils as well as eosinophilia and monocytosis. Injection of recombinant mouse interleukin 3 (rmIL-3) caused a significant increase in peripheral blood eosinophils, neutrophils and lymphocytes. With rmIL-3, however, a remarkable increase was observed in various hematopoietic precursor cells in hematopoietic organs. Both G-CSF and GM-CSF were shown to shorten significantly the period of neutropenia after irradiation and autologous bone marrow transplantation in monkeys. rhG-CSF was demonstrated to accelerate the recovery from neutropenia induced in mice and monkeys by 5-fluorouracil or cyclophosphamide. Human urinary CSF (CSF-HU), which had been reported to stimulate monocyte-macrophage to produce G-CSF, was demonstrated to be effective in accelerating the recovery from neutropenia in patients with various kinds of gynecological and urological malignancies after chemotherapy for their cancers. It also accelerated the recovery from neutropenia after allogenic as well as autologous bone marrow transplantation. These results indicate that CSFs are very effective for the treatment of neutropenia after cancer chemotherapy and bone marrow transplantation.