This assay employs the quantitative sandwich enzyme immunoassay technique. Antibody specific for C4 has been pre-coated onto a microplate. Standards and samples are pipetted into the wells and any C4 present is bound by the immobilized antibody. After removing any unbound substances, a biotin-conjugated antibody specific for C4 is added to the wells. After washing, avidin conjugated Horseradish Peroxidase (HRP) is added to the wells. Following a wash to remove any unbound avidin-enzyme reagent,