The pharmacokinetic behaviour of the non-glycosylated, bacterially-derived recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) and the glycosylated mammalian product was studied after intra and extra vascular administration of a single dose in rodents. Each route of administration gave a different rhGM-CSF concentration-time profile. After extra vascular administration of equivalent doses, a higher peak concentration and faster elimination were observed in the group treated with the E. coli-derived cytokine. The faster elimination resulted in a return to pre-treatment plasma levels after 12 hrs, versus 48 hrs following the administration of glycosylated rhGM-CSF. After intravascular administration, clearance of rhGM-CSF was significantly decreased by the presence of carbohydrates. Non-significant differences in the terminal phase of the biphasic kinetics were found, but the distribution phase was significantly longer for the glycosylated form.