Autoantibodies to the C3 convertase of the alternative pathway of complement, called C3 nephritic factors (C3NeF), cause persistently low C3 in the circulation and production of C3 degradation fragments due to prolonged stabilization of the C3 convertase. C3NeF are associated with glomerulopathy, acquired partial lipodystrophy, and less frequently with increased susceptibility to meningococcal infection. Analysis of C3NeF is an important part of the diagnostic workup of C3 glomerulopathy, but their identification is difficult presumably due to considerable heterogeneity. Therefore it is recommended to use a combination of different analysis methods for the detection of C3NeF. Here we present an ELISA method for detection of C3NeF.