Factors that stimulate the proliferation and differentiation of murine bone marrow cells have been purified from a cloned T cell lymphoma, LBRM-33, and a cloned myelomonocytic leukemia cell line, WEHI-3. These colony-stimulating factors (CSF) have been purified by sequential fractionation by using salt precipitation, gel filtration, anion and cation exchange chromatography, and high pressure liquid chromatography. Both LBRM-33 and WEHI-3 cells secrete a CSF species with similar chemical and biologic properties. This CSF species appears to exist in two forms, termed CSF-2 alpha and CSF-2 beta, both of which stimulate the growth of bone marrow cells in the granulocyte, macrophage, megakaryocyte, mast cell, and erythrocyte lineages, as well as the growth of a CSF-dependent cell line, FDC-P2. These properties of CSF-2 alpha and -2 beta are similar to those reported for interleukin 3, hematopoietic cell growth factor, mast cell growth factor, and persisting cell growth factor. However, LBRM-33 cells secrete another CSF species, not produced by WEHI-3 cells. This CSF species, unique to LBRM cells, is termed here CSF-2 gamma and it stimulates the proliferation of granulocytes and macrophages from bone marrow but does not support the growth of FDC-P2 cells.