Clinicians as well as physiologists have displayed interest in the varied susceptibilities to hemolysis by hypotonic solutions of the erythrocytes of different species of mammals and of human erythrocytes under different pathologic conditions. The variations have been somewhat vaguely ascribed to differences in "osmotic resistance" and in "permeability" due, in turn, to constitutional anomalies and to differences in the age of the erythrocytes. The possibility that differences in the dimensions of erythrocytes might be responsible for differences in susceptibility to hypotonic hemolysis has, however, been mentioned from time to time, and certain correlations in this respect have been pointed out. Chauffard1 said he considered that the wide "resistance span" in hemolytic jaundice was an expression of heterogeneity of cell population in respect to diameter. Vallery-Radot and Lhéritier2 noted that the resistance to hemolysis of the erythrocytes of different species of mammals varied directly with the diameter of the