Six patients exhibiting severe pancytopenia or overt leukemia associated with myelofibrosis after chemotherapy for malignant disease have been investigated by immunologic techniques and ultrastructural cytochemistry. Initially, five patients displayed severe thrombocytopenia contrasting with mild neutropenia and anemia. Bone marrow biopsies showed a clear megakaryocytic proliferation and an excess of immature mononuclear cells. The demonstration of peroxidase activities at the ultrastructural level and immunofluorescence labeling with a panel of monoclonal antibodies, including an antiplatelet glycoprotein Ib and an antiglycoprotein IIb-IIIa complex, on blood or marrow cells, permitted identification of otherwise unidentifiable promegakaryoblastic proliferation. In two patients, the use of an immunoperoxidase technique with an antifactor VIII-R-Ag antibody has allowed direct confirmation of this diagnosis on bone marrow sections. This megakaryoblastic proliferation was not pure and was variably associated with blasts of other cell lines (erythroblasts or myeloblasts). Changes in the population of blasts were observed during evolution in two patients. The sixth patient had a mild thrombocytopenia associated with severe neutropenia and anemia. Bone marrow biopsy displayed a myelofibrosis and immature cells, without megakaryocytic proliferation. Ultrastructural study revealed a pure basophil-mast cell proliferation. In conclusion, in five of six patients with secondary acute leukemia associated with myelofibrosis, a proliferation of promegakaryoblasts was demonstrated using both immunofluorescent and ultrastructural cytochemical techniques.