The origin of cells in the blast crisis of some cases of chronic granulocytic leukaemia (CGL) remains controversial. Difficulties arise from the lack of cytochemical characteristics of differentiation. This report concerns the nature of cells in the blast crisis of a case of CGL in which blast cells exhibited an undifferentiated or lymphoid appearance by light and electron microscopy. The majority (90%) of such cells contained a peroxidase in the endoplasmic reticulum distinct from myeloperoxidase. In addition, some micromegakaryocytes could be recognized among the peroxidase reactive cells, by the presence of typical granules and demarcation membranes. Since this peroxidase exhibited identical characteristics to that of normal megakaryocytic precursors, these blast cells could be identified as megakaryoblasts. These data emphasize the possible megakaryoblastic nature of cells occurring in other cases of CGL blast crisis.