Localization of 5‐Hydroxytryptamine in Blood Platelets: An Autoradiographic and Ultrastructural Study

5‐Hydroxytryptamine (serotonin, 5‐HT) was found by electronmicroscope autoradiography to be localized in dense granules in platelets. Evidence of radioactivity from tritium‐labelled 5‐HT was also found at other sites within platelets, but without evidence of concentration at those sites. Studies on human platelets showed that the dense granules were present in different forms, and structural evidence suggested that the dense granules containing 5‐HT may be derived from alpha granules. After clotting of platelet‐rich plasma, and before the structural integrity of the platelets was lost, radioactivity was present in relation to dense granules, but had been lost from other sites, suggesting firmer binding of 5‐HT to the granules.