Summary. Heavy-large platelets have previously been shown to be more active metabolically and functionally compared to light-small platelets and to be present in increased per cent during conditions of enhanced thrombopoiesis. In the present report, the kinetics of platelet synthesis has been followed by in vivo labelling of megakaryocytes with the amino acid analogue, [75Se]selenomethionine. Kinetic measurements in three volunteer subjects indicated that heavy-large platelets were young platelets and suggested a progression with age of heavy-large platelets to light-small platelets.