The sedimentation differential agglutination technic is a modification of the Ashby method by means of which the concentration of immunologically distinct erythrocytes in a mixture can be determined by observing their volume after agglutination and subsequent sedimentation. The method has been applied to the determination of the efficiency of replacement transfusions, the survival of red cells after blood transfusion, and has afforded a method for the determination of production of erythrocytes under certain conditions. The results indicate an almost complete inhibition of erythropoiesis and the frequent presence of excessively rapid hemolysis in "blast" leukemias. In a number of instances in which simultaneous measurements by the sedimentation differential agglutination and radioactive phosphorus technics were made, good correspondence between the two methods was observed.

ACKNOWLEDGMENTS The author is indebted to Dr. Richard Rosenfield and Miss Grace Ohno who supplied some of the antiserums and performed the replacement transfusions of this study.