Thrombospondin (TSP) is a glycoprotein released from platelets after thrombin-induced aggregation. Endothelial culture studies suggest that it or a similar protein may also be produced by endothelial cells. The origin of the glycoprotein is therefore of some importance. This report concerns the immunohistochemical localisation of TSP within megakaryocytes and platelets. TSP would appear to be a primary platelet protein of megakaryocyte origin, rather than a substance taken up by the platelet in the circulation. The relation of this molecule to the endothelial glycoprotein remains to be established, but our preliminary immunohistochemical studies suggest that the molecule is also located in vascular endothelium.