Urokinase plasminogen activator (uPA), along with its receptor uPAR, is a serine protease that activates plasminogen to plasmin in the blood fibrinolytic system. It is also implicated in events related to cell invasion and migration. Clinical studies have indicated that high uPA levels may elevate the risk for tumor invasion and metastasis. Increased expression and activity can exert potent arthritogenic properties in rheumatoid arthritis patients. Increased uPA activity may be an implication for the pathophysiology of endometriosis. The sensitive quantitative measurement of total human uPA antigen in plasma, serum, culture media or tissue extract samples is easily performed with this 96 well strip format ELISA kit. The concentration of uPA antigen in human plasma has been reported to be 3.5 ng/ml. The assay measures total uPA in the 0.1-50 ng/ml range. Samples giving human uPA levels above 50 ng/ml should be diluted in blocking buffer before use. Human uPA will bind to the capture antibody coated on the microtiter plate. Free and complexed uPA will be detected by the assay. After appropriate washing steps, anti-human uPA primary antibody binds to the captured protein. Excess primary 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 human uPA in the samples. A standard calibration curve is prepared in blocking buffer using dilutions of purified human uPA and is measured along with the test samples. All reagents and standards are provided in these ELISA kits.