The present experiments seem to indicate that horse blood is deficient in both Factors VIII and IX, the concentration of Factor VIII varying from 30 to 60 per cent of the normal human levels, and the concentration of Factor IX varying from 50 to 80 per cent, the corresponding concentrations in 12 normal human males varying from 60 to 150 per cent and from 70 to 150 per cent, respectively.The finding that the recalcification time of horse plasma was considerably longer than that of normal human plasma is of interest, since it is known that only very low concentrations of Factor VIII or Factor IX, namely below 5 per cent, affect the results of the testFinally, by using the glass activation test of Margolis, it was shown that activated equine plasma and activated normal human plasma are equally effective in shortening the clotting time of intact human or equine plasma.