Inoculation of BMA1 cells into BALB/c nude mice formed tumors (BMA1 tumor) that were transplantable into ddY mice, and induced marked granulopoiesis in vivo. Histological study revealed that the tumor was a fibrosarcoma, some parts of which were calcified, and consisted of hemopoietic foci surrounded by adipose tissue. This tumor was regarded as producing CSF in vivo as well as in vitro, since CSF activity was detected in sera of the tumor-bearing mice and tumor extract. Granulopoiesis and splenomegaly developed, associated with an increase of stem cells in the spleen. The number of CFUc and CFUs in the spleen increased to about 91 times and 21 times those of control mice, respectively, whereas the number of stem cells in the tibia did not change significantly. The number of peripheral leukocytes increased to 15 times that of normal mice and amounted to 78% of matured granulocytes. After tumor resection these hematological changes were reversed. The findings suggest that the granulopoiesis in BMA1 tumor-bearing mice may be induced by CSF produced by BMA1 tumor and that the spleen may be a direct target organ of the excessive amount of CSF.