The serum levels of TGF-beta1, measured by solid-phase ELISA, were determined to be significantly augmented in patients with both relapsing remitting (RR) and secondary chronic progressive (CP) MS compared with sex- and age-matched healthy controls. Moreover, in RR MS patients, the blood levels of the cytokine were further augmented either during relapses or, in a rapid but reversible fashion, by s.c. injection with 8 million International Units (MIU) IFN-beta1b. Because TGF-beta1 possesses multiple anti-inflammatory activities, we hypothesize that the increase in its circulating levels in RR and CP MS patients might represent an endogenous anti-inflammatory mechanism aimed at counteracting ongoing immunoinflammatory events, and that IFN-beta may further potentiate this natural defensive apparatus.