That the formed elements of the blood play a part in normal coagulation has long been known. Both leucocytes and platelets have been said to yield substances which contribute to fibrin formation. Leucocytes alone, however, will not coagulate fibrinogen. Cramer and Pringle1 have recently shown that oxalate plasma freed from platelets by filtering through clay filters does not clot on adding an amount of CaCl2 which causes a similar centrifuged but unfiltered plasma to clot in a short time. That platelets disintegrate during coagulation, and that the addition of oxalate preserved them is well known. As early as 1881 Fano2 appreciated the fact that centrifuging was not sufficient to remove all cellular elements, and therefore resorted to filtration through a clay filter.Regarding the effect of other anticoagulants, it has been observed by Buerker3 and by Deetjen4 that the breaking down of platelets in shed blood as observed under the microscope was inhibited by all those substances which can be used in preve...