Human interleukin-3 (IL-3) is expressed in yeast and has a specific activity of 5 x 10(7) U/mg of protein. It exerts functional and proliferative effects on multiple hematopoietic cell lineages including the neutrophil, eosinophil, basophil, monocytic, and thrombopoietic cell lines. IL-3 and granulocyte-macrophage colony-stimulating factor (GM-CSF) share common binding capacities on hematopoietic cells. Each of these agents has entered clinical trials. The clinical experiences with IL-3 alone and in combination with GM-CSF in a Phase I/II trial are summarized in this report.