The absence of a simple and widely applicable test for the measurement of NF activity has hampered the accumulation of evidence bearing on its nephritogenicity. The extensive modification of a screening test for this autoantibody, reported here, has increased the range and precision of the test and made it less laborious. C3b deposited on sheep E by the reaction of NF with NHS forms a C5 convertase which, with addition of rat EDTA serum, leads to hemolysis of the cells proportionate under the right conditions to the concentration of NF in the reaction mixture. The calibration line is straight or slightly concave and passes through the origin. The method detects the activity of both the NF of the amplification loop, NFa, found in MPGN type II, and the NF of the terminal pathway, NFt, found in MPGN types I and III. Interday coefficients of variation ranged from 6.6% to 13.5% and intraday from 7.0% to 12.6%. Although serum C3 levels can be markedly depressed when NF levels are high, C3 levels and NF activity generally correlate poorly. The C3 level could be low and NF absent or, occasionally, NF present with the C3 level normal. NF activity was absent from the stored serum of patients with active SLE, AGN or with an IgA nephropathy.