The count and size of erythrocytes are normally stable. In order to evaluate their ratio in health, some hematological diseases and during exposure to some ecological factors, peripheral blood erythrocytes were studied in 747 subjects (115 healthy controls aged 18-80 years, 56 subjects chronically exposed to ionizing radiation sources, 105 subjects working with aggressive liquids, 117 patients with anemias, and 81 with Hodgkin's disease). No relationship between erythrocyte count and MCV was detected in normal subjects and patients with anemia and Hodgkin's disease. It seems that this relationship is rather specific for hemopoiesis shifts associated with cell cycle delay, as it was detected in patients with B12 and folic deficiency anemias, in subjects chronically exposed to ionizing radiation sources, and in those handling nitrogen oxide in combination with heptyl.