Heparin potentiation of clot lysis by streptokinase was studied in a rabbit model. Clot was initiated in the rabbit aorta with stasis and thrombin and allowed to naturally propagate proximally until stasis met flow. The clot was allowed to age for 1 h before assigning treatment. Fifteen rabbits (group I) were given streptokinase (10,000 IU/h) and 11 rabbits (group II) were given streptokinase (10,000 IU/h) plus sodium-heparin (120 IU/h). Thrombolytic therapy was continued for 5 h. Clot lysis averaged 30% in group I and 70% in group II. Ten of 11 rabbits in group II had more than 50% clot lysis, whereas only 4 of 15 in group I had this degree of lysis. One group II rabbit and four group I rabbits died prematurely; each was noted to have clot propagation at the time of death. While a trend for amelioration of hypofibrinogenemia was observed in the group receiving both streptokinase and heparin, this difference was not statistically significant. We conclude, in the animal model, that thrombolysis by a combination of heparin und streptokinase is more effective than streptokinase alone. Systemic effects are apparently no worse with the combination.