The response of lymphocytes to Concanavalin A (Con A) was measured in patients with the nephrotic syndrome due to minimal change nephropathy (11 patients), focal glomerulosclerosis (15 patients) and membranous nephropathy (21 patients); autologous serum was not used in these studies. There was a significant reduction in lymphocyte transformation in each group of patients compared to normal controls (p less than 0.01 for each group), but there was no significant difference between the individual groups of patients. Impaired lymphocyte transformation to Con A appears therefore to be a general feature of the nephrotic syndrome and is not exclusive to minimal change nephropathy. Measurements of suppressor cell function in 4 patients with minimal change nephropathy, 9 patients with focal glomerulosclerosis and 12 patients with membranous nephropathy were performed at the same time as the above studies. In each group suppressor cell function was decreased, indicating that the impaired lymphocyte response to Con A is not due to increased suppressor cell activity. These findings do not support the hypothesis that an abnormality of lymphocyte function peculiar to minimal change nephropathy is pathogenetic in that disease and not in other causes of the nephrotic syndrome; it seems more likely that the abnormalities described are secondary to the nephrotic state.