We measured serum levels of Facteur Thymique Sérique (FTS) in 56 young cancer patients compared to normal controls. All patients who received immunosuppressive therapy had low age-corrected titres of FTS. Low levels were also found at diagnosis and off therapy. Plasma from 22 patients contained factors capable of inhibiting biological activity of FTS in vitro. The nature of this inhibitor has not been elucidated. No zinc deficiency was found in the patients studied, suggesting that FTS is secreted in its active form. Our study points out the importance of monitoring FTS activity in young cancer patients for its implications on immunological surveillance. The practical applications of thymic hormone therapy in cancer patients are discussed.