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 colonyforming cells per 5 × 104 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%.

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This investigation was supported by The Netherlands Organization for Fundamental Medical Research.

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