"Laborskel-analyzer" (PSL-1) was used to estimate red blood cell volume for the differential diagnosis in 36 polycythemia vera, 20 erythrocytosis patients and 30 normal subjects. Erythrocyte count of polycythemia vera patients (I, IIA and IIB stages) showed a shift to the left (an increase of microcytic fractions), while erythrocyte count++ of 20 erythrocytosis patients (16 of them with a relative erythrocytosis and 4 with an absolute erythrocytosis) were close to erythrocyte count++ of normal subjects. The counting of the percent content of erythrocytic fractions of varying volumes has proved fraction 5 with the erythrocytic volume from 56 microns 3 to 62 microns 3 to have the highest information content. The percent content of this fraction in polycythemia vera fluctuated from 3.9 to 19.5%, and in erythrocytosis it was from 0.9 to 3.0%, that could serve as a criterion for the differential diagnosis of polycythemia vera and erythrocytosis.