With the purpose to elucidate more clearly the actual mechanism of the hematopoietic function of polysaccharides extracted from the calf bone marrow, the author performed bone-marrow tissue culture with addition of polysaccharides at various concentrations, and studied the effects of these substances on the bone marrow. The results are as follows: 1. An appropriate amount of polysaccharides helps to promote the growth of the bone marrow tissue and the wandering velocity of pseudoeosinophils in bone marrow in the coverslip culture. 2. Even in the bone-marrow tissue culture in the fluid medium an appropriate amount of the polysaccharide brings about an increase in the erythrocyte count and hemoglobin content. 3. In other words, the polysaccharides extracted from the calf bone marrow possess the action as to stimulate directly the bone marrow parenchyma, thus promoting the hematopietic function of the marrow. 4. Consequently, the hematopoietic action of these substances is not solely dependent upon the dilatation of the blood vessels in the bone-marrow but also on the promotion of the function of bone marrow parenchyma.