The experiments were conducted on a model of intraorganic growth of Geren carcinoma (GC) in spleen of non-linear mice. It has been shown that titer of thymic serum activity (TSA) decreased sharply in the blood at a stage of settling transplants down (7th day), while the level of an inhibitor for thymic serum factor (FTS) increased in a statistically significant way. At progressive tumor growth, the level of TSA in the circulation raised a little on the 10th day but it was still reduced in comparison with that before inoculation of GC. Traces of the inhibitor for FTS were detected only in 18 and 25 days of tumor growth. We have found resemblance between these substances and those in low-molecular extracts of lymphocytes (LEL). The LEL from cells of the spleen and the thymus of intact rats contained TSA and FTS inhibitor, both of T-cell origin, in ratio 1:1. Production of TCA and the FTS inhibitor was peculiar to immature cortisone-sensitive T-lymphocytes. Anti-FTS serum in vitro completely neutralized TSA in both the blood and an extract of thymocytes but it effected neither the contents of TSA in the LEL of the spleen nor the level of FTS inhibitor in all the samples investigated. The data received testify to an important role of FTS inhibitor in the pathogenesis of tumor progression.