A total of 58 pulmonary lesions from 48 patients were examined for carcinoembryonic antigen (CEA). The 3-layer immunoperoxidase procedure for antigen detection was used with a monospecific anti-CEA antiserum. The control serum was the same antiserum with its specificity removed by affinity chromatography. Normal goat serum was also used as a control. Carcinoembryonic antigen was present in the majority of pulmonary adenocarcinomas and absent in squamous cancers. The major exception was in well-differentiated squamous lesions where CEA was occasionally found in the keratinizing areas. Of special interest was the finding of CEA in all areas of intraepithelial squamous neoplasia studied.