The incorporation of 14C-5HT by platelets was found to be abnormal in two groups of thrombasthenia. In the first group of patients, which have a low platelet fibrinogen content, incorporation was initially faster than in normal platelets. With the exception of one patient who was subsequently discovered to possess an antibody which inhibited ADP-induced aggregation of control platelets, the rate of incorporation of 14C-5HT in group I thrombasthenic platelets reached a maximum and then decreased. In the control platelets incorporation of 14C-5HT was continuous. In a patient of the second group of thrombasthenia, with a low ATP content, incorporation of 14C-5HT was also continuous but was at a much lower level than normal. Results obtained with reserpine suggest that this compound affects the active incorporation of 14C-5HT into platelet granules and, possibly, is associated with a prevention of 5-HT release.