A patent pending mutant of human PAI-1 containing four mutations that stabilize the activity of PAI-1 has been made available. The N-terminus has been additionally modified with the introduction of a 6-X histidine tag. The tag allows for the immobilization of functionally active PAI-1 onto surfaces such as metal chelate microtiter plates or Ni+2 resins. Many new applications for PAI-1 can be envisioned with this unique reagent.