A study of serum and plasma of patients receiving dicumarol reveals that a good correlation exists between the "degradation rate" of "prothrombin-converting factor" in the serum and the concentration of prothrombin in the plasma. As prothrombin concentrations falls, the rate of degradation of "prothrombin-converting factor" becomes slower, while a rise in prothrombin concentration is associated with an accelerated "degradation rate."

ACKNOWLEDGMENT Grateful acknowledgment is made to L. S. Crump, Ph.D., of the University of Rochester Atomic Energy Project for assistence in statistical analysis of data.