Antisera were raised by immunizing rabbits with cloned lines of murine hemopoietic progenitor cells (P cells) that depended on the presence of a specific hemopoietic growth factor, persisting cell-stimulating factor (PSF), for their growth and survival. The unabsorbed antiserum was inhibitory, but after absorption with murine spleen cells and the mastocytoma, P815, significant stimulation of both P cell growth and thymidine incorporation was evident. IgG antibodies isolated from the antiserum by staphylococcal protein A chromatography or further purified by diethylaminoethyl anion exchange chromatography, ammonium sulphate precipitation, and gel filtration using Sephacryl S-300 were responsible for the stimulation. The absorbed antiserum promoted the survival of normal murine bone marrow cells in liquid culture over a four-day period, and the inclusion of IgG antibodies in agar cultures of normal bone marrow promoted the in vitro survival, over a 48-hour period, of cells capable of subsequently generating, in the presence of a source of PSF, colonies of neutrophils, macrophages, and megakaryocytes. It is postulated that the antibodies act by stimulating the PSF receptor on both the factor-dependent cell lines and normal myeloid progenitor cells.