Arginine is added to commercially available recombinant tissue plasminogen activator (rtPA) to promote its solubility. However, arginine is also known to inhibit certain proteases. To explore the possibility that arginine inhibits the highly concentrated rtPA in commercial formulations, we tested the amino acid in assays of tPA function. Although arginine was found to inhibit rtPA cleavage of a chromogenic substrate, it did not interfere with rtPA-mediated fibrinolysis. Nor did it inhibit plasmin activity. However, the coagulant proteases thrombin and Factor Xa were inhibited by arginine. Therefore, the concentration of arginine in commercial preparations of rtPA appears to have an anticoagulant activity but does not retard fibrinolysis.