Granulocyte colony-stimulating factor is a 18 kD glycoprotein hormone which exhibits the following biological actions; (1) amplification and release into blood of bone marrow hematopoietic stem/progenitor cells, (2) enhancement of neutrophil production and stimulation of their release into blood and into tissues, (3) prolongation of neutrophil survival and activation of their various functions, (4) stimulation of myeloid leukemia cell growth. In patients with malignant neoplastic disorders, therefore, its recombinant human form (rhG-CSF) may be an effective strategy for various therapeutic purposes. These include reduction of the risk for infectious complications, more effective killing of malignant cells, and promotion of bone marrow transplantation. We have recently performed several clinical studies of rhG-CSF derived from Chinese hamster ovary cells. Results so far obtained from these studies strongly indicate that rhG-CSF can be safely and efficiently used for the above purposes.