Ten cases of chronic myeloproliferative diseases with myelofibrosis and circulating megakaryoblasts have been studied. The megakaryoblasts were recognized by the presence of megakaryocyte-platelet-related cell surface antigens, detected by an immuno-electron-microscopic method. They showed a tendency to develop from ‘lymphocyte-like’ mononuclear cells to relatively mature megakaryocytes. In these latter cells, the cytoplasm was abundant and contained specific organelles and cytoplasmic blebs. Several characteristic features were observed during the maturation of the megakaryoblasts: (1) cells of the megakaryocytic lineage represented by microforms, (2) dysplastic platelets (giant, nucleated and agranular platelets), (3) the shedding of dysplastic platelets brought about by cytoplasmatic membrane inclusions and (4) naked nuclei.