The behavior of platelets was investigated with an automatic platelet counting method. A pure dilution reaction was observed after infusion of Glucose and Dextran 75. Whereas Dextran 40 caused a considerable diminution of platelets, Haemaccel (Gelatine) decreased platelet concentration significantly, an effect which is reversed within 60 min at the latest. Is is thought that this is caused by platelet aggregation in combination with the filtration-mechanism of the capillary bed and subsequent disaggregation.