This assay employs the competitive inhibition enzyme immunoassay technique. The microtiter plate provided in this kit has been pre-coated with C4. Standards or samples are added to the appropriate microtiter plate wells with Horseradish Peroxidase (HRP) conjugated antibody preparation specific for C4. The competitive inhibition reaction is launched between with pre-coated C4 and C4 in samples. A substrate solution is added to the wells and the color develops in opposite to the amount of C4 in th