A patient with small cell lung cancer (SCLC) whose serum contained high levels of soluble interleukin-2 receptors is reported. Soluble interleukin-2 receptors in the supernatant of cultured SCLC cells obtained from the patient's pleural effusion while he had malignant pleuritis, increased almost linearly from the time of cell seeding. The expression of interleukin-2 receptors (Tac) on the SCLC cells were demonstrated by an immunofluorescence study. However, other lymphocytic markers, including OKT 11, OKT 4, OKT 8, B 1, and B 4, were not found on the cells with the exception of the natural killer cell marker, NKH-1. Southern blot analysis indicated the rearrangement of the T-cell receptor of the cancer cells. Moreover, monoclonal integration of human T-cell leukemia virus type 1 (HTLV-1) provirus in DNA from the cancer cells was also demonstrated. These observations suggest that some SCLC in HTLV 1 endemic areas are associated with HTLV-1.