1 Previous work showed that platelets from patients receiving chlorpromazine (CPZ) have an enhanced aggregation response to 5-HT. This was caused by an action of CPZ on the platelets, a factor borne by the plasma, or both. 2 Therefore we studied 5-HT induced aggregation of platelets from CPZ treated patients resuspended in normal plasma and from normal subjects resuspended in CPZ plasma. 3 Aggregation responses of normal platelets to 5-HT were not enhanced by resuspension either in normal plasma or in CPZ plasma. In addition the enhanced responses of platelets from CPZ treated patients did not revert to normal after resuspension in normal plasma. 4 It is concluded that the enhanced aggregation response to 5-HT seen after CPZ treatment is due to a change in the properties of the platelets rather than a factor in the plasma.