These studies describe an assay of whole blood clot lysis as measured by release of 125I-fibrinogen degradation products. Optimal rates of lysis were obtained at 37 degrees C in 10-12 mM EDTA or 3,8% citrate and 4 u of thrombin/ml. Eighteen normal subjects and eight patients (six with recurrent deep vein thrombosis, one with thrombasthenia, and one with hepatitis and resolving portal vein thrombosis) were studied using this assay. The clots of seventeen of the eighteen normal subjects were 50% lysed at 40 hours. The clots of the patients with venous thrombosis and thrombasthenia did not lyse whereas the clots of the patient with hepatitis, resolving portal vein thrombosis and a high plasminogen activator level (0.32 CTA units) were 100% lysed at 4.5 hrs.