The application of 111In-oxine to platelet labeling has contributed to the understanding of platelet kinetics along three lines: 1. It allows the measurement of new parameters of splenic function, such as the intrasplenic platelet transit time, which has shed new light on the physiology of splenic blood cell handling. 2. It facilitates the measurement of platelet life span in conditions, such as ITP, in which 51Cr may undergo undesirable elution from the platelet as a result of platelet-antibody interaction. 3. It allows the determination of the fate of platelets, that is, the site of platelet destruction in conditions in which reduced platelet life span is associated with abnormal platelet consumption, as a result of either premature destruction of "abnormal" platelets by the RE system, or the consumption (or destruction) of normal platelets after their interaction with an abnormal vasculature. Future research using 111In platelets may yield further valuable information on the control as well as the significance of intrasplenic platelet pooling, on the role of platelets in the development of chronic vascular lesions, and on the sites of platelet destruction in ITP. With regard to the latter, methods will have to be developed for harvesting sufficient platelets representative of the total circulating platelet population from severely thrombocytopenic patients for autologous platelet labeling. This would avoid the use of homologous platelets, which is likely to be responsible for some of the contradictory data relating to the use of radiolabeled platelet studies for the prediction of the response of patients with ITP to splenectomy.