The concentrations of carcinoembryonic antigen (CEA) and the gastrointestinal cancer associated antigen CA 19-9 were studied by radioimmunoassay in serum samples from patients with malignant and benign pulmonary tumours. Elevated levels of either markers were found in 14% of patients with malignant tumours and in none of the cases with benign lesions. Following removal of the tumour, a decline in high CA 19-9 levels was found. The advanced tumours produced the highest serum CA 19-9 concentrations but elevated values were also found in small bronchioloalveolar carcinomas and pulmonary adenocarcinomas. Immunohistochemical staining for CEA and CA 19-9 was performed on routine histopathological specimens. About half of the cases of bronchioloalveolar carcinomas and pulmonary adenocarcinomas were CA 19-9 positive and about 90% were CEA positive. No correlation between intensity of tissue expression and serum levels of CA 19-9 could be demonstrated. However, the bronchioloalveolar carcinomas and pulmonary adenocarcinomas which produced high serum levels of CA 19-9 also stained positively.