The tissue distribution of ⁵¹Cr-labeled platelets and erythrocytes was determined in normal and asplenic rats. Two hours after injection, the distributions in tissues of RBC and platelet ⁵¹Cr were not significantly different except in the spleen which contained platelet ⁵¹Cr (12 per cent), significantly in excess of red cell ⁵¹Cr (1.4 per cent). As labeled platelets were removed from the bloodstream, 80.2 per cent of the injected ⁵¹Cr was deposited in the spleen, liver and marrow of normal animals. During the same interval, radioactivity in all other sites examined diminished to less than 10 per cent of initial values. In splenectomized animals, hepatic uptake of radioactivity was twice that observed in normal animals.

Studies in normal human subjects using external scintillation scanning suggest that similar quantitative considerations apply to man in that, normally, one-third of total platelets are sequestered in the liver and one-third in the spleen while in asplenic individuals two-thirds are destroyed in the liver.

These observations suggest that the majority of platelets are cleared from the circulation by the reticuloendothelial system after becoming "senescent" but they do not elucidate the nature of "senescence" or the circumstances leading to it.