Abstract.The pharmacokinetics of glycosylated recombinant human granulocyte‐macrophage colony‐stimulating factor (rhGM‐CSF) was studied following intravenous (i.v.) and subcutaneous (s.c.) bolus injection of rhGM‐CSF, 8 μg kg‐1employing a sensitive radioimmunoassay. After a single i.v. bolus injection, an initial high serum level of rhGM‐CSF was observed, followed by a rapid decrease that occurred in two phases with a half‐life (t1/2) α of 20.0PM 5 min and at1/2 β of 68.3PM 8 min. Following s.c. bolus injection the absorption was more prolonged. Peak serum concentrations did not occur until about 15–20 h, and were followed by a more protracted elimination than by the i.v. route.In all patients the single rhGM‐CSF injection led to an increase in peripheral white blood cells (WBC), after a temporary drop of 2–5 h duration. The increase in WBC was of longer duration after s.c. than after i.v. bolus treatment. Since the subcutaneous administration leads to prolonged serum concentration of rhGM‐CSF and prolonged increase in peripheral WBC, it seems preferable to i.v. bolus injection, and as effective as