Platelet life span and sites of platelet sequestration were studied on 20 occasions in 15 patients with idiopathic thrombocytopenic purpura (ITP) using 51Cr-labelled cells and external scintillation scanning. In 16 cases when platelet concentrations were greater than 5000 per cu.mm., labelled cells were destroyed predominantly in the spleen. Only in four patients with platelet levels less than 2500 per cu.mm. was significant hepatic sequestration observed. In the latter group, half-disappearance times of labelled cells were extremely short, ranging from 2 to 9 minutes. The data suggest that in ITP, as in other clinical and experimental situations where circulating cells are sequestered following their reaction with antibodies, lightly sensitized cells are removed from the circulation at a relatively slow rate by the spleen, while more heavily sensitized cells are liable to rapid destruction in the liver. Thus, in ITP, the site in which labelled cells are sequestered appears related to the severity of the disease at the time of study, rather than to any qualitative difference in the nature of the disease among individual patients.