Two different adhesion patterns of blood platelets with different sensitivity to adenosine were demonstrated by means of reflection contrast microscopy with consecutive image analysis and cell affinity chromatography. High-performance liquid chromatography revealed that thrombin-induced serotonin release of adenosine-sensitive platelets was lower than that of adenosine-resistant cells. Our results indicate platelet heterogeneity and suggest that the platelets with lower adenosine sensitivity may be actively involved in the early interaction between platelets and injured endothelium.