Carcinoembryonic antigen (CEA) is one of the most important tumour markers in the management of human carcinoma, including lung cancer. So far, however, because of the nonspecificity of anti-CEA antibodies, it remains unclear whether the experimental measurements of CEA expression really reflect genuine CEA. In normal lung, nonspecific cross reacting antigen (NCA) has been described as a major component of CEA-related antigens. Recently isolated CEA and NCA cDNA clones enabled us to analyse CEA and NCA expression of in vivo tumour specimens and tumour cell lines at mRNA levels. NCA-specific mRNA (but not CEA-specific mRNA) was detected in all normal lung tissues examined. Of 21 lung cancer tissue specimens, nine expressed both NCA and CEA and five expressed only NCA. Of 16 tumour cell lines, two expressed only NCA and one expressed both NCA and CEA, although its level of CEA mRNA was weaker than that of NCA mRNA. Therefore, CEA-related mRNA expression was always accompanied by NCA mRNA expression; there were no cases of CEA mRNA expression alone. These findings suggest that NCA is a major member of the CEA-related gene family expressed in lung cancer.