HITHERTO no satisfactory method for the determination of the life of blood platelets has been described. Recently a method has been reported for determining the life of erythrocytes using diisopropylfluoro-phosphonate labelled with phosphorus-32¹. This method was based on the irreversible combination of this compound with esterases present on the red cell membrane. Since thrombocytes have also been shown to contain esterase, the same method suggested itself as a way of measuring the life of these cells. It seemed reasonable to assume that conditions similar to those described for red cells would be met. This implies that the decay of radioactivity associated with platelets would be entirely due to the destruction of these cells and therefore an index of their normal breakdown. This would only be true if no re-incorporation of labelled material in newly formed platelets were to take place. This assumption seems justified, since it has been shown that the only demonstrable metabolite of diisopropylfluorophosphonate in the human body is diisopropylphosphate. This compound is not further metabolized but quickly excreted in the urine. The labelled diiso-propylfluorophosphonate used was synthesized by Mr. Oosterbaan (M.B.L., Rijswijk) by a method to be reported in the near future.