Daily administration of granulocyte colony-stimulating Factor (G-CSF) results in progenitor cell mobilization with maximum blood levels achieved after 4–7 days. In this study the short-term effects of glycosylated G-CSF at a dose of 5 μg/kg s.c. were determined so as to allow optimization of the timing of progenitor cell collection. In the first study involving 20 normal volunteers, a significant fall in neutrophil count and G-CSF levels was observed 2 h after the G-CSF injection. To investigate this phenomenon serial measurements were made in a further six volunteers after the 6th daily injection of G-CSF. A fall in the neutrophil count occurred which was maximal at 1 h and recovered to baseline within 3 h. There was also a fall in CD34+ cells (P = 0.034), GM-CFC (P = 0.025) and BFU-E (P = 0.066) and recovery to baseline levels took 4–12 h. We conclude that glycosylated G-CSF should not be given immediately prior to stem cell collection.