The main principle of enumerating platelets in automated hematology analyzer is electric resistance system. However, there have been an increasing number of instruments that can enumerate platelets by optical system. The use of a flow cytometry (FCM) employing monoclonal antibody has been under study for the enumeration of platelets in recent years. At its meeting held in April 2000, the International Society for Laboratory Hematology (ISLH) decided on a protocol to study the possibility of using, as reference method, a monoclonal antibody-employed FCM of enumerating platelets. In our present study, we obtained platelet count by the monoclonal antibody-employed method proposed by ISLH, and compared the result with platelet count obtained with an automated hematology analyzer XE-2100 (Standard counter at Scientific Division, Sysmex, Kobe, Japan) employing electric resistance system, and another platelet count obtained by optical system.