The aim of the present study was to investigate the in vivo effects of granulocyte colony‐stimulating factor (G–CSF) on neutrophil (PMN) function. G–CSF was administered once daily as s.c. injection for 6 d (d1–6) to healthy male volunteers. PMN migration (modified Boyden chamber), chemiluminescence (CL), adherence to nylon fibers and phagocytosis of IgG‐ and IgG‐C₃‐coated particles were investigated before (d1), during (d2, d5) and 3 wk after G–CSF 7.5–10 μg/kg/d (n=12). PMN surface expression of adhesion‐ and Fcγ‐receptors was measured on d1, d5, d8 and 3 wk after G–CSF 3–5 μg/kg (n=12). Results obtained after G–CSF were compared to baseline using Wilcoxon's signed rank test. G–CSF induced PMNs showed a significantly (p<0.05) decreased chemokinetic response (d5) as well as a reduced chemotaxis towards zymosan activated serum, FMLP and IL‐8, respectively. Chemotaxis was reduced both at d2 and d5. Neutrophil adherence, phagocytosis and luminol‐enhanced CL increased, whereas G–CSF had no effect on lucigenin‐enhanced CL. G–CSF (3–5 μg/kg) caused an enhanced expression of CD11b, CD18, CD35, CD64 (FcγRI) and CD32 (FcγRII), respectively. We conclude that neutrophils produced in response to G–CSF have a reduced chemotaxis but an enhanced adherence and phagocytic capacity. G–CSF in vivo does not stimulate the respiratory burst.