The frequency distribution of erythrocyte volumes of animals and man, determined electronically, showed a skew toward large volumes. Simple mathematical and computer analyses of these curves seemed to reveal the presence of two populations of erythrocytes with Gaussian distribution. One consisted of cells of relatively small volume and the other of cells of large volume. Study of avian erythrocytes revealed that in these species the two populations were sufficiently different volumetrically that their distributions did not completely overlap and their presence was not obscured in the total distribution curves. Analysis of pathologic states showed that, in most, the volumetric distributions shifted toward large volume as the disease developed. In severe erythroblastosis, for example, only the large population was demonstrable. Consequently, the larger-volumed population appeared to be composed of reticulocytes and young erythrocytes. These studies and those in progress indicate that this technic may afford another useful dimension to investigative and therapeutic hematology.