CD61 (GPIIIa) is a glycoprotein found on megakaryocytes, platelets and their precursors. CD61 antigen plays a role in platelet aggregation and also as a receptor for fibrinogen, fibronectin, von Willebrand factor and vitronectin. Clone 2f2 will prove useful in detecting neoplastic platelet precursors, normal platelets and most cases of megakaryocytic leukemia.