Plasma protease C1 inhibitor (aka C1 esterase inhibitor or SerpinG1) is a single-chain, 478 amino acid glycoprotein that regulates classical complement activation and the contact activation or intrinsic coagulation pathway. This 105 kDa serine protease inhibitor has a C-terminal serpin domain with unique stabilizing disulfide bonds and highly glycosylated N-terminal domain. C1 inhibitor irreversibly inactivates C1r, C1s, MASP1, MASP2, plasma kallikrein, thrombin, plasmin, tPA, and Factors XIa and XIIa. Genetic deficiency in plasma C1 inhibitor results in Type I hereditary angioedema. The sensitive quantitative measurement of total human C1 inhibitor in human plasma and serum is easily performed with this 96 well strip format ELISA kit. The normal human plasma concentration of C1 inhibitor is 0.2 mg/ml. The assay measures human C1 inhibitor in the 1-500 ng/ml range. Samples giving human C1 inhibitor levels above 500 ng/ml should be diluted in blocking buffer before use. A 1:10,000-1:40,0000 dilution for plasma is suggested for best results. Human C1 inhibitor will bind to the affinity purified capture antibody coated on the microtiter plate. Free and complexed C1 inhibitor will bind to the plate. After appropriate washing steps, horseradish peroxidase labeled polyclonal anti-human C1 inhibitor antibody binds to the captured protein. Excess antibody is washed away and TMB substrate is used for color development at 450nm. A standard calibration curve is prepared along with the samples to be measured using dilutions of human C1 inhibitor. Color development is directly proportional to the concentration of total C1 inhibitor in the samples. All reagents and standards are provided in these ELISA kits.