Plasma, in an amount corresponding to 0.05 gm fibrinogen/Kg of body weight, was administered during one hour to defibrinogenated dogs subjected to extracorporeal circulation. Four blood filters included in the extracorporeal circuit were removed one by one at 30-minute intervals. The prefilter pressures rose gradually during the whole experiment. Scanning electron microscopy of the filter surface revealed equal amounts of deposits before, during and after the infusion of plasma. Only minimal amounts of fibrin were found on the filter surfaces by immunoelectrophoresis. Mean arterial pressure, arterial blood gases, fibrinogen concentration, platelet count and leukocyte count were followed before, during and after plasma infusion and showed no significant changes. Thus, it is possible to administer limited amounts of plasma to defibrinogenated dogs subjected to extracorporeal circulation.