Metachromatic staining cells with mast cell and basophil characteristics were grown from normal human bone marrow, both in co-culture with a human mast cell-like strain (HBM-M) and when maintained in the presence of conditioned medium derived from this cell strain (HBM-M-CM). In co-culture with HBM-M, >25% of the bone marrow cells stained metachromatically with toluidine blue from day 20-40 compared to <10% in control cultures. Half the cells were stained by the basophil-specific antibody Bsp-1 and 50% bound IgE. In the presence of 50% HBM-M-CM, up to 60% of marrow-derived cells were positive for toluidine blue, while 15-36% of cells bound Bsp-1. Conditioned medium from HBM-M cells did not contain IL-3, IL-5, or GM-CSF but did contain approximately 0.1 ng/ml stem cell factor. HBM-M cells appear to secrete an unidentified growth factor(s) capable of promoting the production and/or differentiation from normal human bone marrow of metachromatic staining cells in liquid culture.