We previously identified a receptor for granulocyte colony-stimulating factor (G-CSFR) on platelet membranes, and reported that G-CSF enhanced ADP-induced platelet aggregation. Here, we investigated the priming effect of G-CSF on the hemostatic system in healthy volunteers given G-CSF. Following the administration of rhG-CSF (10 micrograms/kg for 30 min div) to 10 healthy volunteers, we found a significant elevation in the maximum platelet aggregation rate induced by ADP or collagen, thromboxane B2 level and amount of thrombin-antithrombin III complex. The D-D dimer and plasminogen activator inhibitor-1 showed no significant changes. These observations indicate that G-CSF administration may induce hypercoagulability in susceptible subjects. Therefore, patients or donors at risk of thrombosis or hypercoagulable state should be followed carefully after G-CSF administration.