The number of bone marrow granulocyte-macrophage progenitor cells (CFU-C) and fibroblastoid colony-forming units (CFU-F) was significantly smaller in acute leukemia patients. To analyze the mechanism by which CFU-C and CFU-F were reduced, the study on murine myeloid leukemia was carried out. In murine leukemia, bone marrow CFU-C and CFU-F decreased in number in correlation with the proliferation of leukemic cells. Murine leukemic cells suppressed the growth of CFU-C and CFU-F in the mixed culture of leukemic cells and normal murine marrow cells. Furthermore, leukemic cell-conditioned medium inhibited the growth of CFU-F. The results show that murine leukemic cells themselves and/or humoral factors produced by them give inhibitory effects on the growth of not only CFU-C but also CFU-F.