Mouse monoclonal antibodies were raised to the storage and secreted forms of eosinophil cationic protein (ECP), and were used to study the presence of activated eosinophils and secreted ECP in the tissues of patients with a variety of allergic diseases. Immunocytochemical localization was shown with alkaline phosphatase-linked second antibodies, and fast-red substrate. Deposition of a red reaction product indicated sites in tissues where eosinophils had become activated, and where secreted ECP was present. Activated eosinophils and secreted ECP were found together in (1) skin lesions of patients with chronic urticaria, (2) gut lesions of patients with eosinophilic gastroenteritis and ulcerative colitis, and (3) tissues containing granulomas in patients with allergic granulomatosis and vasculitis – the Churg and Strauss syndrome. These results show the value of these techniques for determining the sites of eosinophil activation and secretion in allergic diseases. They support the suggestion that ECP may be involved in the development of these tissue lesions.