Platelet mean life span (MLS) and platelet production were studied in 11 patients with idiopathic thrombocytopenic purpura (ITP) before and after institution of corticosteroids.

Platelet MLS was greatly reduced before treatment. A prolongation of MLS was observed in every patient during corticosteroid therapy but there was in no case a normalization of platelet MLS.

Platelet production was significantly above normal in untreated ITP and did not change significantly during corticosteroid treatment. The partial compensation of the platelet count during treatment was due to an increase of platelet MLS and a continued unchanged elevated platelet production.

Recovery of infused labelled platelets was greatly reduced in ITP and was closely related to platelet MLS. During corticosteroid treatment the recovery values increased significantly.

The survival curve in ITP consists of a rapid component terminated 15 min after infusion and a second one with a slower disappearance rate. The percentage platelet recovery is dependent on the magnitude of the first component. Due to limitations of splenic blood flow the rapid initial platelet destruction is mainly accomplished by the liver. In untreated ITP with a high rate of initial platelet destruction a greater amount of radioactivity would accumulate in this organ. Accordingly external surface counting of radioactivity has shown an increase in the spleen to liver ratio during corticosteroid treatment.