This report describes a relatively simple test for C3 nephritic factor (C3NeF) activity utilizing immunofixation electrophoresis to quantitate the production of the breakdown product C3c. Tests performed on plasmas from 22 control patients and 93 renal patients having biopsy diagnoses other than basement membrane dense deposit disease (BMDDD) yielded negative results for C3NeF activity. Tests performed on plasmas from three patients with BMDDD were positive for C3NeF activity, yielding values significantly higher than those for the control group and the patients with other renal diseases. The test can be performed on specimens collected in EDTA to prevent in-vitro C3 degradation during storage or transport at ambient temperature.