The lysis rates of 125I cross-linked plasma fibrin clots have been investigated following immersion in various buffered concentrations of streptokinase (SK), prior to (called the SK → plgn regimen) and after (called the plgn → SK regimen) enrichment with 2.0 U of plasminogen (plgn, lys-type). The SK → plgn regimen usually achieved total clot lysis with the most active lysis occurring following enrichment in 125–130 IU SK solutions, while only partial lysis was observed during the plgn → SK procedure. The effect of SK concentration was marked during the SK –plgn regimen while, during the plgn → SK treatment, its lytic rate effect never exceeded a factor of 2.0 over a wide concentration range of SK (8.0–20,000 IU/ml. The availability of plasminogen to perfuse the activator-rich clot seemed to be the prevailing influence on clot lysis and lysis of clots in plasma was inhibited by serum inhibitors, SK antibodies and pl