Serum levels of soluble interleukin‐2 receptors (sIL‐2R), carcinoembryonic antigen (CEA), alpha‐fetoprotein (AFP), β‐chorionic gonadotropin (β‐HCG), pregnancy‐specific glycoprotein (SP₁), and β₂‐microglobulin (β₂M) were taken in 92 patients with primary lung cancer and 43 controls. The mean value of sIL‐2R in the cancer group was twice as high as that of the controls (P less than 0.001) and the highest values were observed in those with small cell carcinoma (SCC) (P less than 0.0001). Of the cancer patients, 51.1% had CEA values higher than the cutoff level of 5 ng/ml. Extended‐disease patients had a higher percentage of increased CEA values than those with limited disease. Adenocarcinoma (ADCC) and SCC groups had the highest percentages of increased CEA levels. There was no significant difference between the groups for β‐HCG, AFP, SP₁, and β₂M, and intermarker correlation was not seen. The results suggest that sIL‐2R and CEA may be useful in monitoring the extent of disease and possibly indicate the histologic subtype, thus having a bearing on treatment and prognosis.