High levels of soluble lymphocyte antigens have been described in a large number of tumors and, particularly, in hematopoietic neoplasms. As previously reported, many antitumor immune responses are IL-2 dependent: clinical observations indicate that a worse survival in advanced tumor patients is related with a decrease of soluble IL-2 levels. A soluble form of CD8 has been described: as found in Hodgkin's disease and acute lymphoblastic leukemia, sCD8 levels have a prognostic value. To explain the significance of these soluble molecules in solid tumors, we a) determinated sIL-2R and sCD8 in 84 patients; b) correlated the expression of p55 chain of IL-2R and CD8 antigen on the cell-surface of peripheral lymphocytes to sIL-2R and sCD8 levels; c) analyzed endogenous IL-2R levels in patients with lung cancer. An increase of sIL-2R was found in 82% of cases, while high levels of sCD8 were observed in 32%; no correlation was observed between sIL-2R and the expression of p55 on the surface of peripheral lymphocytes: IL-2 levels in patients with NSCLC were significatively reduced, when compared to healthy controls, with an inverse relationship between endogenous IL-2 concentration and sIL-2R levels. Whatever may be the physiopathological mechanism of the increase of sIL-2 observed in solid tumors, this rise may contribute to the immunodepression correlated to neoplastic disease. Therefore, higher levels of sIL-2R/IL-2 ratio has a negative biologic prognostic significance. We think that determinating CD8 antigen in the serum can offer a more sensitive and specific measurement of activation of suppressor/cytotoxic T-lymphocytes.