Kinetics of lysis of human plasma clots immersed in plasma were studied in vitro at 37 degrees C under the influence of recombinant staphylokinase, single-chain urokinase-type plasminogen activator (scu-PA), and their simultaneous and consecutive combinations. Staphylokinase and scu-PA caused concentration- and time-dependent lysis of the clots; 32 nM staphylokinase and 75 nM scu-PA separately caused 50% lysis in 4 h. At these equally effective concentrations staphylokinase in 4 h induced a significantly lesser exhaustion of the plasma plasminogen, alpha(2)-antiplasmin, and fibrinogen than scu-PA. Combinations of staphylokinase (<30 nM) and scu-PA (<75 nM) rendered synergic thrombolytic action on the clots. The synergy of thrombolytic action was more pronounced on the simultaneous addition of the two agents than on their consecutive addition, scu-PA 30 min after staphylokinase. In 4 h after the addition, staphylokinase (25 nM) or scu-PA (15 nM) induced 24% and 2% lysis, respectively, whereas the simultaneous and consecutive combination of the same concentrations of these agents induced 58% and 50% lysis, respectively. The simultaneous combination of 15 nM staphylokinase and 15 nM scu-PA resulted in maximal 3.8-fold increase in the thrombolytic effect as compared to the expected total effect of the individual agents. Synergic combinations of the two agents caused lesser exhaustion of plasma plasminogen, alpha(2)-antiplasmin, and fibrinogen as compared with the expected total effect of these agents used separately. Thus, simultaneous and consecutive combinations of staphylokinase and scu-PA in a relatively narrow range of their concentrations possessed synergistic fibrin-selective thrombolytic action on the plasma clot in vitro.