The effect on platelets of rhG-CSF was studied in 20 healthy volunteers with the thrombometer, a specially developed device which is described in detail. Additionally, conventional aggregation tests were performed. Low doses of rhG-CSF enhance functional platelet activity, as shown by significant acceleration of the occlusion of the thrombometer channel. Similar results were found in conventional aggregation tests utilizing collagen for induction. At G-CSF concentrations of 0.1 and 1.0 ng/ml the time to response was significantly accelerated and the maximum response was observed in a higher proportion of platelets. However, the second phase of aggregation induced by epinephrine was significantly inhibited by 1.0 ng/ml G-CSF. The activation of platelets may be beneficial in thrombocytopenia, but it may also increase platelet turnover and platelet loss. Further investigation is needed to clarify the mechanism by which G-CSF exerts its effects on platelets.