A serum assay of CA-50, CA-125, CA-19.9, ENOLASE (NSE) and CEA was performed in 65 patients with primary lung cancer, 63% of whom had visceral metastases, with the following histological distribution: squamous carcinoma (40), adenocarcinoma (9), large cell carcinoma (4), oat cell carcinoma (12). A raised CEA level was detected in 37% of cases, compared with 40.6% for CA-50, 44.6% for CA-19.9 and 40.6% for CA-125 and only 26.6% for NSE. Overall, at least one marker was positive in 83% of cases. Statistical analysis of the different markers (correlation analysis and principal component analysis) demonstrated the existence of a strict correlation between the levels of CA-50, CEA and CA-19.9 (p less than 0.01). ENOLASE was more frequently elevated in oat cell carcinomas (42%) than in non-oat cell carcinomas (30%), but the difference was not significant. In contrast, CA-125 appears to be a good marker for non-oat cell carcinomas (51%) (p less than 0.05). The authors also found a correlation between the presence of a raised level of CA-50 (p less than 0.05), CA-125 (p less than 0.02) and especially CEA (p less than 0.001) and the presence of visceral metastases. The initial survey of disseminated lung cancers should include the assay of at least 4 of these markers in order to obtain a reliable serum tracer in more than 2/3 of cases.