We have observed that decreased plasma levels of C3 in the serum of three children with poststreptococcal acute glomerulonephritis (PSAGN) at the time of presentation were associated with the presence of C3NeF activity in purified serum IgG from the patients. C3NeF activity was determined using a sensitive assay measuring the ability of patients' IgG to stabilize a preformed cell-bound alternative pathway convertase complex. C3NeF activity of patients' IgG decreased within weeks as plasma levels of C3 progressively returned to normal values. C3NeF activity became undetectable within 1-4 months following normalization of plasma C3 levels. Our observations suggest that early alternative pathway-dependent hypocomplementaemia, a cardinal feature of PSAGN, is mediated by the transient expression of C3NeF autoantibody activity by patients' IgG.