The distribution of 111In-labelled platelets following intravenous bolus injection has been studied using a gamma camera and computer system. Liver uptake, which accounted for about 10% of the dose, was completed between 6 and 10 min after injection. Blood pool and splenic 111In, which accounted for the remainder of the dose, reached constant levels simultaneously about 20 min after injection. The kinetics of splenic uptake are consistent with a two compartmental model in which circulating and splenic platelets are in dynamic equilibrium with each other. From analysis of the kinetics, splenic blood flow and the mean transit time of platelets through the spleen have been calculated in normal subjects and in patients with haematological disorders. Blood flow, which was about 200 ml per min in normals, tended to increase with increasing spleen size. Transit time was not dependent on spleen size; it was about 10 min in all but one of the subjects.