The sensitive measurement of functionally active rat uPA is easily performed with this 96 well strip format ELISA kit. Functionally active uPA present in citrated plasma, serum, culture media or tissue extracts will bind to functionally active PAI-1 coated on the microtiter plate (patent pending). Only free active enzyme will react with the PAI-1 on the plate. Inactive or complexed enzyme will not be detected. After appropriate washing steps, anti rat uPA primary antibody binds to the uPA.