The anticomplementary activity (ACA) of the sera of 81 patients with primary glomerular disease was studied. ACA was mesured by a complement consumption test, and the ACA of alternaive pathway was measured by a homolysis inhibition test, using rabbit red blood cells in EGTA buffer. In acute glomerulonephritis, the ACA of the sera with high levels of antistreptokinase and γ-globulin was significantly higher than that of the sera with low levels. The ACA of the sera of sclerosing glomerulonephritis patients was significantly higher than that of the sera of patients in other histlogical categories based on light microscpy. There were no relationships between immunofluorescence findings in the glomerulus and the ACA of the sera of patients with primary glomerular disease. In chronic glomerulonephritis, the ACA of the sera correlated with the levels of BUN, S-creatinine, PSP (15 min.) and Ccr significantly (p<0.001). The sera from uremic patients showed predominantly high ACA and contained heat stable factors (m.w.; more than 25000 daltons) witch were able to activate the classical complement pathway. There observations suggested that the anticomplementary factors in acute glomerulonephritis might be immune complexes and those in uremia might be unknown uremic toxins.