An 11-month-old boy was transferred to our hospital because of fever and bleeding tendency on March 13, 1998. Laboratory studies showed a white blood cell count of 43,360/microliter with 75% blasts, a hemoglobin concentration of 8.4 g/dl, and a platelet count of 23 x 10(3)/microliter. Surface marker analysis with a flow cytometer revealed that only 21% and 11% of the blasts, respectively, were positive for CD41 and CD42b. Treatment with a permeabilizing agent apparently increased the reactivity of the blasts with anti-CD41 monoclonal antibody (MoAb), which can recognize IIb independently of IIIa. However no significant differences were observed in reactivity with anti-CD41 MoAb (which recognizes the IIb/IIIa complex) anti-CD61 MoAb and anti-CD42b MoAb before or after fixation. Blasts positive for platelet peroxidase were observed by electron microscopy, thus confirming the diagnosis of acute megakaryoblastic leukemia. We concluded that the detection of intracellular antigens is useful for the quick diagnosis of acute megakaryoblastic leukemia characterized by low surface expression of megakaryocytic lineage antigens.