Six patients with giant platelet syndrome were examined: four with Bernard-Soulier syndrome (two were asplenic); one with hereditary thrombopathic thrombocytopenia; and one with May-Hegglin anomaly. Autologous platelets were labelled with In-111-oxine and in vivo redistribution and sites of sequestration measured with quantitative imaging. In Bernard-Soulier syndrome platelet survival was normal or moderately shortened; platelet turnover was decreased only in the two patients with thrombocytopenia. In the patients with thrombopathia or May-Hegglin anomaly, platelet survival and turnover was moderately decreased. In those patients with normal-sized spleens, the mean splenic platelet pool consisted of 35.5% of the platelet mass, i.e. normal. The intrasplenic transmit time of the megathrombocytes was prolonged. Splenic blood flow was within normal limits. There was a marked accumulation of platelets in the liver at equilibrium: 15.5-58.8% of whole body radioactivity (normal 9.6 +/- 1.2%). This finding is unexplained. The final sites of sequestration of platelets were mainly in the liver and spleen, similar to that seen in normal subjects. We conclude that there is no inverse relationship between cell size and splenic platelet transit time. Platelet size therefore does not determine the size of the splenic platelet pool. The size of the platelets also does not seem to affect the sites of sequestration at the end of their life span.