INTRODUCTION Certain constant results have been reported and much interest awakened within recent years in the study of the fragility of erythrocytes. It is not expected that similar findings will prevail in work on leukocytes, which are regarded as approaching more nearly to typical human tissue in structure and function than erythrocytes. Academically, the observation and measurement of particular reactions of leukocytes as an example of living tissue are of definite interest, but from the clinical standpoint special attention is aroused by the finding of certain reactions by Mauriac, Carbonat and Moureau that are claimed to indicate the prognosis of many diseases. Mauriac, Carbonat and Moureau1 have done the primary work of importance in the field of leukocyte fragility. An outline of their work follows: Mauriac,2 in 1916, devised a technic for the determination of the resistance of leukocytes by exposing blood to a hypotonic aqueous solution of 0.206 per