Bacterially synthesized human granulocyte colony-stimulating factor (G-CSF) and granulocyte macrophage colony-stimulating factor (GM-CSF) have been studied to determine if they could prevent or reduce the neutropenia caused by chemotherapy. Our studies suggest that 10 micrograms/kg/day of G-CSF administered as a continuous subcutaneous infusion abrogates the neutropenia associated with a standard dose of melphalan. G-CSF produced a rapid increase of neutrophil levels, was well-tolerated, and was associated with only one frequent adverse effect: bone pain. GM-CSF, administered in doses ranging from 3 to 15 micrograms/kg/day subcutaneously, appeared to be useful in abrogating the neutropenia associated with chemotherapy, producing elevations in neutrophils, eosinophils, and monocytes. Although GM-CSF was relatively well-tolerated, several adverse effects, including bone pain, rashes, and fluid retention, were observed. The initial dose of GM-CSF in some patients produced a reaction that was characterized by hypoxia.