Suppressor cell activity (SCA) was analyzed in 8 patients with focal glomerular sclerosis (FGS) and 11 patients with chronic proliferative glomerulonephritis (CGN). We have assessed the ability of peripheral blood lymphocytes (PBL) stimulated by concanavalin A (Con A) to inhibit the proliferative response of normal allogeneic lymphocytes by both Con A and phytohemagglutinin (PHA). It was found that the FGS patients with nephrotic syndrome (NS) had significantly increased levels of suppression index when compared to the values obtained with normal controls. In contrast, the mean suppression values in the PBL from FGS patients in remission and CGN patients with or without NS, whether the mitogen used was Con A or PHA, were similar to those of the control subjects. Thus, the majority of FGS patients with NS demonstrated an alteration in Con A-induced SCA. The possible significance of these phenomena in the pathophysiology of FGS is discussed.