The contribution of platelet-plasminogen and the effect of inhibitors of u-PA or t-PA on plasminogen activation by platelets. Platelets were incubated with S2403 either in the absence of added plasminogen1,2 or in the presence of 1 μmol/L plasminogen3-7 without1,3 or with 5 U/mL of thrombin.2,4-7 Some platelets5-7 were premixed with 25 μg/mL antihuman u-PA IgG, 1 mmol/L amiloride, or 25 μg/mL antihuman t-PA IgG before the addition of plasminogen, S2403, and thrombin. The samples were assayed for plasmin activity after a 5-hour incubation. The data shown are mean values of duplicate determinations from two experiments. The duplicate determinations were within 5% of each other.