C3 nephritic factor (NEF), an IgG autoantibody to the alternative pathway C3 convertase, is usually measured by crossed immunoelectrophoresis (CI) but recently a reliable haemolytic assay (HA) was described by Rother (1982). This method is more specific than CI because it is negative in sera with immune complexes, SLE and sera incubated with IgG aggregates. The haemolytic assay is sensitive enough to detect NEF antibody in serum from patients with only slightly low C3 levels and NEF negatives by CI. The haemolytic assay is easy to perform and reproducible, the interassay coefficient of variation being 10.7% compared to 64% in the CI. The intra-assay coefficient of variation in CI was 28% compared to 5.5% in the haemolytic assay. The haemolytic method enabled us to study the kinetic effects of NEF on C3b.Bb bound to sheep erythrocytes, and the lysis mediated by ShE.C3b.Bb.NEF complex. Also the C and NEF binding to sheep erythrocytes was studied.