Using the method of concanavalin A (Con a) inducible suppressor activity the peripheral mononuclear cells (PMC) of 17 patients with Mesangial IgA nephritis, 13 patients with idiopathic mesangial proliferative glomerulonephritis but with no IgA deposits on immunofluorescence (Non-IgA nephritis) and 18 healthy subjects were studied. DNA synthesis was measured by incorporation of 3H Thymidine. The mean suppression index (S.I.) of 0.94 +/- 0.08 (SEM) in the IgA nephritic patients was significantly different from that of the non-IgA nephritic patients (0.67 +/- 0.05) (P less than 0.01) as well as the group of normal healthy controls (0.64 +/- 0.05) (P less than 0.0025). The data show that patients with IgA nephritis have abnormal suppressor cell function and suggest an autoimmune basis in the pathogenesis of IgA nephritis.