Prolonged infusion of streptokinase at the customary dose of 100 000 u/h results in undesired plasminogen depletion in many patients. This can be avoided by adaptation of the streptokinase maintenance dose to the presumed rate of plasminogen synthesis of each individual patient. The practicability of this approach was tested in 52 patients who had streptokinase therapy of 3 to 9 days duration for deep vein thrombosis. Twice daily measurements of thrombin time and fibrinogen concentration were performed for immediate clinical surveyance and dosage adjustments. These led to a change from the original 100 000 u/h in most patients: in 65% the dose was reduced and in 10% it was increased. By this measure excessive plasminogen depletion was avoided in 88% of the patients. In them the final maintenance dose ranged from 40 000 to 150 000 u/h. Side effects were similar to those reported for the standard dosage scheme, and clinical results were good with a phlebographic success rate of 91% in recent and 65% in subacute or chronic deep vein thrombosis.