Facilitation of t-PA–catalyzed activation of plasminogen by polymerizing fibrin monomer. Enhancement of t-PA–catalyzed plasminogen activation by fibrin monomer was measured in 180 μL of the reaction mixture composed of acid-solubilized fibrin monomer (0.2 mmol/L), plasminogen (1.0 μmol/L), t-PA (4 U/mL), and S-2251 (0.3 μmol/L) as described in Materials and Methods. Plasmin generation was measured by monitoring A405 nm at 2-minute intervals.