Con A induced suppressor cell activity was evaluated in 10 patients with Systemic Lupus Erythematosus and in 20 normal subjects. The suppressor activity, calculated according to Shou et al. (1976), was markedly reduced in SLE patients (mean value 45.2% vs 72.9% P less than 0,01). No statistical difference was evidentiated between the patients with mild and those with severe disease, though the last ones showed the lowest values. Low degree of suppression was found related to low hemolytic complement activity (P less than 0.02) but not to anti-DNA antibody titre or immune complex presence.