Patients with defective plasminogen activator inhibitor protein (PAI-1) or with PAI-1 deficiency can experience hemorrhage as a result of a hyperfibrinolysis. In these patients, a normal thrombus forms, but endogenous lysis is unchecked as tissue plasminogen activator is unopposed. Treatment includes anti-fibrinolytic agents, including oral tranexamic acid. Another treatment option is the administration of PAI-1, but this serpin rapidly inactivates itself. We have developed a mutant plasminogen activator inhibitor with a very long half life (VLHL PAI-1, t1/2>700 h). Here we investigate VLHL PAI-1 effects in the blood of PAI-1 deficient mice, as a model of human disease. Using a thrombelastograph, we found that blood clots of PAI-1 knockout mice were lysed much more quickly than wild type mice. Additionally, blood clots had less shear elastic modulus strength than clots of normal animals. VLHL PAI-1 treatment of PAI-1 deficient mice extended or prevented thrombus lysis and increased clot strength in a concentration dependent fashion. These two parameters determine the extent of thrombus growth and regression; thus, further testing is anticipated to confirm the effectiveness of plasminogen activator inhibitor with a very long half life in an in vivo model and we hope that this protein can be effective in human PAI-1 deficiency disorder.