This document describes a procedure for analysis of platelet aggregation (1-6). Platelets are small (~2 μ m) anuclear cells obtained by fragmentation of megakaryocytes. Platelets, also known as thrombocytes, play a key role in hemostasis. Abnormal platelet counts and function may lead to either bleeding or thrombosis. Assessing nanoparticle effects on human platelets in vitro allows for quick screening of their potential anticoagulant or thrombogenic propertiesmediated by direct effects on platelets.