The purpose of this study is to present the results of C3 and C4 measurements in 21 patients with acute poststreptococcal glomerulonephritis (A.P.S.G.N.) in order to show the incidence of two possible mechanisms of complement activation suggested in recent studies. C3 and C4 were determined using commercially available immunodiffusi on plates (Behringwerke). In all the patients, the measurements were done during the first two weeks of the disease and followed up thereafter. Three patients had normal C3 and C4 concentrations. The other 18 patients showed an initial reduction of C3 levels and six of them also had an initial reduction of C4 levels. Clinical and laboratory parametres showed no differences between these three groups, which could explain the different patterns of C3 and C4 concentrations.