To date, G-CSF is the most favoured cytokine administered for PBSC mobilization because of its great efficacy and lack of serious toxicity. Recently, a pegylated filgrastim (pegfilgrastim) has been introduced. Attachment of the polyethylene glycol (PEG) moiety reduces renal excretion and masks proteolytic cleavage sites resulting in elevated G-CSF serum levels for up to 14 days after a single injection. As single-dose pegfilgrastim had similar effects in the prophylaxis of chemotherapy-induced neutropenia as the daily administration of the unconjugated drug, its capability for the mobilization of haematopoietic stem and progenitor cells has been assessed and presented to be at least equal to that of conventional G-CSF. Administration of pegfilgrastim following high-dose therapy and autologous blood SCT (BSCT) shortened the time to myeloid recovery as seen in conventional G-CSF. Plasma G-CSF levels were about 1 log higher with pegfilgrastim, but in the setting of autologous BSCT this did not translate into a faster haematopoietic recovery. Only few data exist on the biological effects of pegfilgrastim. Still, these data suggest that pegfilgrastim-stimulation results in different functional properties of haematopoietic stem and progenitor cells compared with G-CSF.