Serum IgG, IgA, IgM, IgE, C3 and C4 were measured in 13 patients with minimal change (MC) glomerulonephritis and 10 with the nephrotic syndrome (NS) due to other forms of glomerulonephritis. The tests were repeated in all patients with MC glomerulonephritis when they went into remission. Serum IgG was reduced, IgM, IgE and C3 were raised while serum IgA was within the normal range when the patients were nephrotic. Changes in serum immunoglobulins and complement components were not specific to MC glomerulonephritis and these parameters reverted towards normal when the NS went into remission. Elevated C3 levels probably reflected increased hepatic protein synthesis since C3 correlated significantly with serum cholesterol. There was a tendency for serum IgE concentrations to positively correlate with the total dose of prednisolone required to bring the NS to remission.