Study has been made of the influence of addition of human NH2 terminal glutamic acid plasminogen (Glu-Plg) or human NH2 terminal lysine plasminogen (Lys-Plg) to normal citrated plasma upon the rate of lysis of fully crosslinked plasma clots in the presence of single or two chain urokinase type plasminogen activator (scu-PA/tcu-PA) or tissue plasminogen activator (t-PA). The specificity of any thrombolytic property was evaluated by measurement of plasma fibrinogen levels. Lys-Plg added to a concentration of 20% of normal plasma plasminogen caused 5 to 6 fold increase in the extent of lysis observed at 6 hours by 100 units/ml of scu-PA and with a small increase in fibrinogenolysis. Glu-Plg added at 20% of normal level had no influence on thrombolysis but at 50% of normal caused increased thrombolysis with rapid depletion of plasma fibrinogen. An apparently synergistic effect of addition of tcu-PA on scu-PA activity was increased by addition of plasminogen (e.g. addition of 20% Lys-Plg increased the lysis rate 4 to 5 fold over the first hour equivalent to an increase of potency of approximately three to four fold). Addition of plasminogen up to double the normal plasma concentration was observed to have no influence on clot lysis in the presence of t-PA. Plasminogen potentiated the rate of lysis by scu-PA/t-PA synergic mixtures with an approximately 1.5 to 1.9 fold increase in potency. Potentiation occurred without increase in the depletion of plasma fibrinogen.(ABSTRACT TRUNCATED AT 250 WORDS)