P-cell-stimulating factor (PSF) (also termed interleukin 3) produced by the T-cell clone A3-37.4, the T-cell hybridoma 123, the T-lymphoma EL4, spleen cells, and the myelomonocytic cell line WEHI-3B had a similar apparent mol. wt. and in each case eluted from a Waters C18 silica column at a concentration of acetonitrile of 38%. Both the PSF from the T-cell clones and from WEHI-3B stimulated the in vitro growth of cloned T-dependent mast cells and of colonies from normal bone marrow cells. The T-cell sources--but not WEHI-3B--also produced an additional, distinct hemopoietic growth factor that stimulated the growth of colonies of neutrophils and macrophages but did not support the growth of P cells. This factor was termed T-cell granulocyte-macrophage colony-stimulating factor (T-cell GM-CSF). T-cell GM-CSF eluted from a C18 silica column at an acetonitrile concentration of 41%, differing in this respect from both PSF, which eluted at 38% acetonitrile, and the GM-CSF produced by endotoxin-stimulated mouse lungs.