As part of phase I/II clinical trials of granulocyte colony-stimulating factor (G-CSF), the pharmacokinetics was studied. To determine the optimal way of abrogating the neutropenia caused by melphalan, patients received G-CSF and melphalan on several schedules. The half-life (t 1/2) of elimination of G-CSF was in the range 1.3 to 4.2 hours and was prolonged at higher doses, suggesting that one clearance mechanism becomes saturated at doses greater than 10 micrograms/kg, When a continuous subcutaneous (SC) infusion was administered for five days, a rapid reduction in serum G-CSF levels occurred during the last two days of the infusion, indicating that an additional clearance mechanism was induced. When a continuous infusion of G-CSF was administered after melphalan, serum G-CSF levels remained constant for a longer period of time but did decrease during the second phase of a biphasic neutrophil response. In another clinical trial, G-CSF was administered after high-dose chemotherapy and autologous bone marrow transplantation (ABMT). In these patients, the G-CSF levels did not decrease while the patients were neutropenic. These results show that increased neutrophil levels are associated with increased clearance of G-CSF. This may be one of the negative feedback mechanisms involved in maintaining neutrophil homeostasis in normal and disease states.