We investigated the biological characteristics of 'eosinophilic' leukaemic blasts from a patient with eosinophilic blast crisis of chronic myelogenous leukaemia (CML). Southern blot analysis of the bcr gene demonstrated the identical single rearranged band in both the leukaemic blasts and mature eosinophils. IL-3, IL-5 and GM-CSF significantly enhanced proliferation and differentiation of these leukaemic blasts, and only eosinophils were generated from in vitro cultures. Those eosinophils expressed the B3A2 chimaeric messenger RNA seen in the leukaemic blasts. These observations indicate that eosinophil progenitors may be involved in the development of eosinophilic blast crisis of CML and that IL-3, IL-5 and GM-CSF act on the leukaemic eosinophil progenitors.