Seven rabbits were injected with 217 and 108 units (plasma equivalents) of factors II and IX per kg. body weight, as components of a preparation of human prothrombin complex. Another group of 34 animals received half that amount. Fibrinogen, factors V and VIII, and platelets were found to be reduced, suggesting intravascular generation of thrombin. These factors were only slightly affected by injections of the solvent alone (0.13 M sodium chloride and 0.01 M sodium citrate). Of the animals injected with the prothrombin complex, 34% died, whereas none of the control group died. The mortality was higher (82%) when the complex was injected into the jugular vein through a catheter than when it was injected directly into the marginal vein of the ear (17%). The results, however, were not affected whether the complex was injected before or after the surgery for catheterization.