Glutathione (GSH), a component of the antioxidant defence system, plays a role in autoimmunity and the complement system is often responsible for tissue damage in autoimmune diseases. The aim of this study is to evaluate the effects of GSH on the complement system. The complement system was examined in the normal human sera (NHS) of 30 healthy subjects. Increasing quantities of GSH (1, 2, 10, 20 mg) were incubated in 1 ml of each NHS. The mixtures were evaluated for complement activities (THC, CPA and APA) and for the presence of cleavage fragments of activation of C3 and B. GSH was also incubated with human complement in the presence of classical and alternative pathway activators. The results showed an inhibitory effect of GSH on the complement system starting from a dosage of GSH≥1 mg/ml. Indeed, when NHS was incubated with GSH at such dosage, a significant reduction of the complement activities THC, CPA, and APA was observed (P<0.0001, P<0.005, P=NS, respectively), and no cleavage fragments of C3 or B were found. Further analysis demonstrated that the inhibition was exerted on C3-9 and to a lower extent on classical and alternative pathway C3-convertases. Our results indicate that GSH is capable of inhibiting the complement system. These findings are relevant for the design of interventions aimed at modulation of GSH metabolism to inhibit complement-mediated damage in autoimmune diseases.