Abstract: A composition is provided comprising patelets having a combination of the following modifications: a) a modification of the protein constituents of the platelets which is obtained or obtainable by genetic modification of platelet precursor cells; b) incorporation of a detectable label into said platelet; whereby the functions of modification a) and b) are mutually independent. Further, methods of determining platelet functions, notably aggregation and adhesion to endothelial cells are provided. Further, a novel method of preparing transgenic or modified platelets is provided.