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Platelet rich plasma was prepared with soybean trypsin inhibitor (SBTI) as anticoagulant, to preserve normal Ca++-concentration, and subsequently tested in an aggregometer. Shear stress alone, induced by stirring, resulted in platelet aggregation and disaggregation. Such platelets showed only minor aggregational response on the addition of ADP, indicating that the platelets had been partly desensitized towards ADP-stimulation. When the stirrer was started at the same time as the addition of 1.6 microM ADP, the aggregation velocity was related to the speed of the stirrer. Disaggregation always started 1.5 min after the stimulus. Platelet shape change was apparently not influenced by the stirring. Since platelets from SBTI-anticoagulated blood could be aggregated by stirring alone, we examined the effect of centrifugation of whole blood at 50 xg for 3 and 10 min. Signs of transient aggregation and release of alpha-granule content were respectively observed.