The presence of the soluble form of the interleukin-2 receptors (sIL-2R) was evaluated in the serum of 21 patients with small cell lung carcinoma (SCLC) and 37 patients with non-small cell lung carcinoma (non-SCLC) by means of an enzyme-linked immunosorbent assay. The sIL-2R level was measured serially in patients with SCLC both during and after therapy. The mean serum level of sIL-2R in patients with SCLC was 3.8 times higher than that of 47 healthy controls and was 1.9 times higher than in 37 patients with non-SCLC. Six patients with SCLC had very high levels of sIL-2R, ranging from five to 52 times the mean level observed in normal controls. Tumor cells in the pleural fluid of the patient with highest levels were positive with monoclonal antibodies to IL-2R (CD25), NKH-1, OKDR, and OKT9. A longitudinal study in this patient showed a good correlation between tumor activity and sIL-2R levels. Also, the sIL-2R levels decreased in patients responding to therapy. These results suggest that some SCLCs secrete sIL-2R and that the serial measurements of the serum sIL-2R levels can be used as a noninvasive tumor marker in this disease.