Biologic and biochemical properties of a human basophil-like cell promoting activity (BaPA), which induces growth of metachromatically staining cells from normal bone marrow cells in a liquid culture system have been examined. In order to study this T cell factor, an assay was developed based on the intracellular histamine content of the cultured human bone marrow cells. Many lymphokines, including granulocyte-macrophage colony-stimulating factor, granulocyte colony-stimulating factor, interleukin 1 alpha and 1 beta, interleukin 2, and interferon-alpha and gamma, did not exhibit any significant activity in the assay. By employing this assay, BaPA was purified approximately 500-fold from lectin-stimulated spleen cell-conditioned medium. BaPA has a molecular weight of 23,000 on high performance liquid chromatography gel filtration and displays isoelectric points between 5.8 and 7.3. It is heat stable up to 80 degrees C for 30 min and resistant to 6 M guanidine hydrochloride, whereas it is rather sensitive to sulfhydryl reagents. BaPA has no stimulating activity on mouse bone marrow cells.