Hematological values of the peripheral blood in the mice transplanted with methylcholanthrene-induced fibrosarcoma were examined. Values for red blood cells and platelets reduced after tumor transplantation. Values for white blood cells increased with the percentage of neutrophils being increased significantly. Phagocytic activity of neutrophils from the tumor-bearing mice was not reduced. The cultured fluid of the tumor used in the experiments possessed strong colony stimulating activity to bone marrow cells.