Immunohistochemical methods were used to identify A2F4 monoclonal antibodies raised by immunization of animals with a human lung adenocarcinoma cell line in 17 lung adenocarcinomas and five pulmonary metastases of other origin. The antigen identified by the antibodies was registered in 53% of lung adenocarcinomas. Localization of the pulmonary antigen was compared with that of the carcinoembryonic antigen. The expression of the antigens proved different.