1. Extracts with physiological saline solution were obtained from about 20 species of invertebrates and seaweed. Tosyl-L-Arg-MeOH hydrolysing and fibrin plate lytic activity were detected in the invertebrates Stichopus japonicus, Crassost gigas, Tapes japonica, and Kintai-gai as well as the seaweed Codiales codium. 2. These activities were all labile against heat (at 65 degrees C for 1 hr). Except for the extract from Stichopus japonicus, lytic activities against fibrin plates with and without plasminogen were similar. 3. The extract from S. japonicus showed plasminogen activating potency as well as the existence of urokinase (UK) activity enhancing factor. 4. On the other hand, the extract of the seaweed Hizikia fusiformis showed a strong UK inhibiting activity. 5. A fraction of fibrinolytic enzyme was obtained from the extract of S. japonicus by absorption to the celite affinity chromatography. It was orally administered to rabbits at a dosage of 40 mg/kg/day. 6. Fibrinolytic activity was determined periodically on the eugloblin fraction of plasma samples collected from these animals. 7. As compared with the pretreatment value, the activity increased about 2 times (P less than 0.01) and 3 times (P less than 0.005) after 4 and 8 weeks, respectively, of the treatment. 8. After 8 weeks of treatment, the kidney of treated rabbits was extracted with 2 M KCl. The activity of tissue plasminogen activator (free-type TPA) was revealed to be enhanced significantly (P less than 0.001) in the extracts. 9. The fibrinolytic enzyme increased in the blood was recognized by zymography to be mainly the UK type plasminogen activator with mol. wt of 53,000.