A cell line showing many characteristics of immature mast cells has been established from the peripheral blood of a patient with mast cell leukemia. Cultured cells contain low levels of histamine, are stained metachromatically by toluidine blue, and contain chloroacetate esterase, aminocaproate esterase and tryptase activities. The cells lack T and B lymphocyte, as well as myeloid cell markers, and do not possess IgE receptors. Solid tumors of metachromatically positive cells have been successfully induced and serially passed in nude mice using 5-azacytidine transformed cells. This cell line may be useful for future studies of mast cells and their constituents.