The biological activity of elements is determined by what place they occupy in the D. I. Mendeleyev periodic system. Investigations by a number of scientists have established the participation of elements in the fourth period (horizontal row) in the hemopoietic process. The most active are the intermediate elements-manganese, iron (macroelement), cobalt, nickel and copper. Each of them acts on different hemopoietic links, supplementing one another. Cobalt facilitates the new formation of reticulocytes and the utilization of reserve iron. Nickel acts like cobalt. Copper participates in the synthesis of hemoglobin. Manganese accelerates the maturing of erythrocytes. Presented are facts obtained under experimental and clinical conditions, theoretical generalizations and practical recommendations.