The reaction patterns in 80 adenocarcinomas of the lung were examined with PAP-method using a monoclonal antibody against keratin and one against carcinoembryonic antigen (CEA) and a polyclonal antiserum against CEA. Almost all tumors showed a positive reaction to the antibodies which, however, varied quantitatively. Even though a reliable correlation of positive immunohistochemical reaction with the different light microscopical types was not possible according to WHO subtypes and degrees of differentiation, specific localization of the reaction within the tumor cells was seen with increasing differentiation. There was no correlation between the immunohistochemical reactions and 14 clinically measured plasma CEA levels. The plasma CEA level not only depends on CEA production by the tumor but also on other factors.