Bone marrow from normal BALB/c mice, mice with myeloid leukemia induced by Soule myeloid leukemia virus, and mice with virally induced mammary carcinoma was cultured in semisolid agar. Bone marrow from either leukemic or mammary tumor-bearing mice produces more clones in vitro in the presence of a specific colony-stimulating factor. However, in all cases, the myeloid progenitor cells have similar requirements for the colony-stimulating factor. The optimum condition for growth in all instances is 7% fetal calf serum + 7% horse serum + 7% tryptose phosphate broth. Decrease in the concentration of these three constituents has a less drastic effect on in vitro proliferation of bone marrow cells from leukemic mice. Some cells from Soule virus-induced leukemias even grew in the absence of serum. The combination of suboptimal amounts of serum and colony-stimulating factor is used as a tool for detecting cells with altered growth characteristics in bone marrow of leukemic mice. During the progression of the leukemia, there is an increase in the amount of transformed colony-forming cells per 5 X 10(4) bone marrow cells. The increase is already noticeable 4 weeks after inoculation, when no clinical signs of the leukemia are present, and reaches a maximum of about 20%.