Serum interleukin 2 (IL-2) and soluble interleukin 2 receptor (sIL-2R) were serially determined in 44 patients with multiple myeloma (MM) by radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA). Serum IL-2 and sIL-2R levels of 24 cases in stage I and II were not different from those in normal individuals. However, serum IL-2 level of 20 cases in stage III was significantly lower than that of normal individuals and of MM patients in stage I and II, while the serum sIL-2R level was significantly increased. The changes in serum IL-2 and sIL-2R level could reflect the severity of the disease.