Factor X is a disulfide linked two-chain glycoprotein zymogen and is the precursor of the coagulation enzyme Factor Xa. Factor X is activated by Factor IXa in complex with Factor VIII, calcium and phospholipids during the intrinsic pathway and by Factor VIIa in complex with Tissue Factor, calcium and phospholipids during the extrinsic pathway of the coagulation cascade. The sensitive quantitative measurement of total mouse Factor X antigen in citrated plasma, serum, cell culture media, or tissue extract samples is easily performed with this 96 well strip format ELISA kit. Factor X, Xa, and Xa in complex with inhibitors or cofactors will be detected by the assay. The concentration of Factor X in normal plasma is 10 ug/ml. The assay measures total mouse Factor X in the 2.5-1000 ng/ml range. Samples giving mouse Factor X levels above 1000 ng/ml should be diluted in blocking buffer before use. A 1:100 dilution for plasma is suggested for best results. Mouse Factor X will bind to the capture antibody coated on the microtiter plate. After appropriate washing steps, biotin labeled anti-mouse Factor X primary antibody binds to the captured protein. Excess primary antibody is washed away and bound antibody is reacted with peroxidase conjugated streptavidin. Following an additional washing step, TMB substrate is used for color development at 450nm. Color development is proportional to the concentration of Factor X in the samples. A standard calibration curve is prepared using dilutions of purified Factor X and is measured along with the test samples. All reagents and standards are provided in these ELISA kits.