Peripheral blood platelets were examined in 7 patients who underwent bone marrow ablation with high dose chemotherapy and total body irradiation prior to bone marrow transplantation (BMT). Platelets were studied for 9 days, by which time the platelet counts had fallen to levels at which platelet transfusion support was necessary. Values for111Indium autologous mean platelet lifespan were similar to those reported in thrombocytopenic patients by other authors. During the period of study, there was no significant change in mean platelet volume or platelet nucleotides. The results indicate that in man, platelet size and platelet nucleotides are not affected when platelets age in the peripheral circulation.