Eosmophil cationic protein (ECP) is a protein specific to the granules of human eosinophil granulocytes. ECP is highly cationic and may damage tissue if not inactivated. Heparin is a highly anionic substance present in mast cells and basophil granulocytes. The presentin vitrostudy shows that ECP can inactivate the anticoagulant activity of heparin probably by the formation of a complex between the two molecules. This function may be of importance for the microenvironment of allergic diseases where secretion of heparin may promote penetration of mast cell products through tissues. Also this may constitute one mechanism whereby the cytotosic action of ECP is neutralized.