Serum levels of soluble IL-2 receptors (sIL-2R) by an ELISA method in 28 patients with sarcoidosis and 16 healthy controls were studied, and the source of sIL-2R was further examined. sIL-2R in serum was significantly higher in sarcoidosis than in controls. In sarcoidosis sIL-2R in serum significantly correlated with serum ACE level, and was significantly higher in stage II or III patients than in stage O patients. sIL-2R in supernatants of cultured monocytes (Mo) and alveolar macrophages (AM) was significantly higher in sarcoidosis than in controls. sIL-2R in supernatants of cultured T lymphocytes obtained from peripheral blood or BALF was barely detectable in sarcoidosis, while it was undetectable in controls. Furthermore, sIL-2R in serum was significantly correlated with sIL-2R in supernatants of cultured Mo and AM. These results indicate that sIL-2R in serum is an useful index of the disease activity of sarcoidosis, and may be mainly derived from IL-2R on Mo and AM.