We describe the unique characteristics of leukaemic basophils from a patient with chronic myelogenous leukaemia (CML). The leukaemic cells were immature basophil-like blasts and expressed CD4, CD7 and HLA-DR in addition to CD13 and CD33. Both immunoglobulin and T cell receptor genes were retained in germline configurations. Interleukin-1 (IL-1) or granulocyte-macrophage colony-stimulating factor (GM-CSF) as well as IL-3 or IL-4 enhanced the proliferation and differentiation of leukaemic cells and only basophils were generated from in vitro culture. These results suggest that basophil progenitors expressing CD4, CD7 and HLA-DR may be involved in the development of basophilic crisis of CML and that both IL-1 and GM-CSF may act on basophil progenitors as well as IL-3 or IL-4.