The myeloid growth factors are a promising new class of therapeutic agents with the potential for broad clinical application. Four recombinant myeloid growth factors, granulocyte macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), macrophage colony-stimulating factor (M-CSF), and interleukin (IL)-3 are available for clinical trials. GM-CSF has been studied in leukopenia related to acquired immunodeficiency syndrome (AIDS), aplastic anemia, and myelodysplasia, as well as in patients receiving chemotherapy and those undergoing autologous bone marrow transplantation. In these trials, GM-CSF was demonstrated to increase the number of neutrophils, eosinophils, and monocytes with corresponding bone marrow changes. Toxicity is dose-related, and maximum tolerated dosages are in the range of 16 to 32 micrograms/kg/day. The effects of daily subcutaneous administration of GM-CSF appear to be similar to those of intravenous (IV) administration. G-CSF, studied mainly in the treatment of neutropenia following cytotoxic chemotherapy, was found to decrease the duration of severe neutropenia as well as the risk of infections. G-CSF causes prominent increases in neutrophil levels without affecting eosinophils or monocytes. Associated toxicity is minimal, and subcutaneous administration is efficacious. Preliminary evidence suggests that G-CSF may also have a therapeutic role in indolent lymphoid neoplasms complicated by neutropenia. Administration of natural M-CSF to patients receiving chemotherapy and those with chronic childhood neutropenia has shown modest neutrophil increases. Preclinical data on IL-3 suggest that this agent increases neutrophils, eosinophils, basophils, reticulocytes, and possibly platelets.