Acute megakaryoblastic leukaemia remains an uncommonly recognized disorder, usually identified by electron microscopy or with monoclonal antibodies. Using two monoclonal antibodies (HP1-1D identifying glycoprotein IIB/IIIA and W1-23 identifying von Willebrand factor) we have studied during a 12 month period all cases of acute leukaemia with FABL2,M1 or undifferentiated morphology presenting in our cooperative group 'Grupo de Hematólogos de Puebla' in México. Six of 21 FAB L2, M1 or undifferentiated leukaemias were classified immunologically as megakaryoblastic. Five of these patients had myelofibrosis and two had normal platelet counts but abnormal platelet aggregation function. All six patients were treated with low-dose cytosine arabinoside (10 mg/m2) administered subcutaneously, twice daily in 21 d courses). Haematologic response was achieved in five cases while the sixth died before completing one course. Three patients relapsed within the first month after completing the chemotherapy and died. Two patients remain in remission 11 and 15 months after initial treatment. It is our impression that the prevalence of acute megakaryoblastic leukaemia has been under-estimated, and that low-dose cytosine arabinoside treatment may be of value in its management.