Immunoglobulin synthesis by pokeweed mitogen-stimulated lymphocytes from 62 nephrotic patients and 18 healthy controls was studied. Defective IgG production was observed in patients with the nephrotic syndrome related to minimal change disease, mebranous glomerulonephritis and membranoproliferative glomerulonephritis. The mean values of in vitro IgG synthesis were 20%, 61% and 32%, respectively, of that obtained in the control group. At the same time, serum IgG levels were significantly decreased in each group of patients. In minimal change disease, reduced IgG production and serum levels were fully reversible after recovery (off steroid therapy). The data indicate that a cellular defect of antibody production is a common and eventually transitory phenomenon associated with the acquired hypogammaglobulinemia found in patients with a variety of glomerulopathies.