A new myeloid cell line (KU812) was established from a patient with blastic crisis of chronic myelogenous leukemia. His blasts were morphologically characteristic of immature basophils and basophil colonies were grown in agar culture of the blood mononuclear cells. Suspension culture of his blood cells was continued for more than 2.5 years. The KU812 cells morphologically showed a fine reticular nuclei with nucleoli, and some of them contained metachromatic granules with toluidine blue (TB) staining. These granules were positive for astra blue (AB) staining. Immunological marker studies revealed that there were no lymphoid characters except Fc receptors. The KU812 cells grew colonies in in vitro agar cultures, which were proved to be composed of basophils by TB staining and AB staining. Cytogenetical analysis showed marked aneuploidy and was positive for the Philadelphia chromosome (Ph1). The cell lysate was proved to contain histamine. These data suggest that KU812 is a cell line from leukemic basophil precursors. This is the first human basophil cell line, and KU812 will be useful in clarifying the mechanism of basophilic differentiation of the stem cells.