The authors investigated the blast cells obtained from two patients with acute myelofibrosis with the use of recently developed immunochemical and ultrastructural markers. They intended to examine the expression of megakaryoblastic, erythroblastic, myeloblastic, and monoblastic properties in these cells. Respectively, 20% and 15% of the blasts from patient 1 and patient 2 expressed a megakaryoblastic phenotype as determined by reactivity with a polyclonal platelet glycoprotein antisera (PGP). However, 55% and 73% of these patients' blasts also exhibited myeloid/monocytic properties. Ultrastructural studies early demonstrated findings consistent with the presence of myeloblasts, monoblasts, and erythroblasts, as well as undifferentiated agranular blasts. These findings demonstrate the existence of a disorder in which blast cells simultaneously express properties associated with multiple hematopoietic lineages. This disorder is characterized by impaired terminal differentiation.