Plasminogen activator inhibitor type 1 (PAI-1) is involved in the regulation of the blood fibrinolytic system. Increased plasma levels of PAI-1 are implicated in the impairment of fibrinolytic function and may be associated with thrombotic diseases. Levels of PAI-1 increase with age and are elevated in conditions such as normal pregnancy and sepsis. The sensitive quantitative measurement of total rabbit PAI-1 antigen in plasma samples is easily performed with this 96 well strip format ELISA kit. The concentration of PAI-1 antigen was reported to be 20.5 ng/ml in rabbit plasma and 11.8 ng/ml in rabbit serum. The assay measures total PAI-1 in the 0.07-67 ng/ml range. Samples giving rabbit PAI-1 levels above 67 ng/ml should be diluted in blocking buffer before use. It is important to ensure a platelet free preparation of plasma as platelets can release PAI-1. Rabbit PAI-1 will bind to the capture antibody coated on the microtiter plate. Free, latent, and complexed PAI-1 will be detected by the assay. After appropriate washing steps, anti-rabbit PAI-1 primary antibody binds to the captured enzyme. Excess antibody is washed away, and bound antibody is reacted with the secondary antibody conjugated to HRP. Following an additional washing step, TMB substrate is used for color development at 450 nm. Color development is proportional to the concentration of PAI-1 in the samples. A standard calibration curve is prepared using dilutions of purified PAI-1 and is measured along with the test samples. All reagents and standards are provided in these ELISA kits.