The application of quantitative biological and biochemical methods to the study of the blood platelets has led to revolutionary developments in our understanding of these elusive structures.1 Until recently platelets were often regarded as non-cellular in nature, although it has long been known that they are of cellular origin. Tocantins2 in his important review in 1938 expressed the thought which was then current: "It is preferable to think of the platelet not as a cellular element but as a transition between the microscopically amorphous elements of the blood (proteins, fats, etc.) and definitely cellular constituents such as erythrocytes or leukocytes." The platelet has been looked upon as a fragile envelope containing a group of mysterious and unidentified substances involved in the coagulation of blood. Now, however, there is an accumulation of evidence which clearly demonstrates that platelets are indeed living cellular structures, non-nucleated to be sure, but comparable