The kinetics of pooling of platelets in the spleen may be used to measure spleen function. However, pooling of platelets in other organs, especially the liver, may affect these kinetic measurements. We have therefore compared, in man and baboon, the kinetics of 111In-labelled platelets and 111In-labelled red cells in the liver and spleen during the first 60 min after reinjection. This was determined in vivo with a scintillation camera and computer-assisted image analysis. Organ radioactivity was expressed as a percentage of that of the whole body. In both humans and baboons, the spleen accumulated many more platelets than red cells. Also, the red cells equilibrated more rapidly. The presence of a splenic platelet pool was thus confirmed. In contrast, the percentages of labelled platelets and labelled red cells in the liver were similar in both species. In both species, the sum total of labelled platelets in the circulation, i.e. recovery, and that quantified in the spleen at equilibrium, were equal to approximately 100%. These results confirm the presence of the exchangeable splenic platelet pool, and indicate that there are no significant exchangeable platelet pools in either the liver or any other organs.