This study is a retrospective comparison of the results in 25 low-dose, intra-arterial streptokinase and 12 low-dose intra-arterial urokinase infusions for thromboembolic disease. Intra-arterial streptokinase was successful in 50% of infusions and was marked by significant abnormalities in the coagulation criteria. There was a high incidence of major and minor bleeding (48% overall), which could be attributed to systemic effects of the drug. Urokinase was successful in 100% of infusions, and showed no significant effects on systemic coagulation criteria. There were also fewer complications during urokinase infusion. The average pharmacy cost for a course of intra-arterial streptokinase was $165, while urokinase cost $1142. Despite the significant difference in expense, the increased efficacy and safety of urokinase make it the preferred agent for intra-arterial infusion. Theoretical reasons for the increased effectiveness of urokinase are discussed.