Abstract-The effects of latamoxef, cefamandole, carbenicillin and cefotaxime on fibrinolytic system in rats were examined under feeding of an ordinary diet or a vitamin K-deficient diet for eight days. No obvious change was observed in UK induced plasma clot lysis time, euglobulin lysis time and antiplasmin level in plasma. These findings of the in vitro and ex vivo studies suggest that these antibiotics cause no enhancement of fibrinolytic activity.