Human bone marrow cells grown in liquid culture in the presence of conditioned medium from concanavalin A stimulated peripheral blood mononuclear cells, from mixed lymphocyte reactions, or from an osteogenic sarcoma tumor line gave rise to basophils which were maximal in number at 1-3 weeks. Basophils had a multilobed nucleus, contained large cytoplasmic granules of variable size which stained metachromatically with acid, but not neutral toluidine blue, were negative for chloroacetate esterase, and did not contain human mast cell tryptase. In long-term cultures, mast cells were detected after 5-6 weeks. Mast cells had cytoplasmic granules that stained with acid and neutral toluidine blue, were positive for chloroacetate esterase, and contained human mast cell tryptase. It, therefore, appears that in liquid cultures even in the absence of feeder layers, human bone marrow cell cultures can give rise initially to basophils, and then to mast cells.