Fluid from crevices of clinically healthy gingivae was studied for its content of plasmin, plasminogen activator, proactivator of plasmingoen and plasminogen. Unheated and heated Astrup fibrin plates were used. The crevicular fluid was collected by applying tips of triangles of filter papers into the interproximal parts of the crevices. The crevicular fluid contained plasmin, plasminogen activator, pro-activator of plasminogen and plasminogen. The plasmin activity of the fluid corresponded approximately to a trypsin solution of 15 ug of trypsin per ml. The activator was thermostabile and was inhibited by E-amino caproic acid and S-amino valeric acid. It was checked that the plasmin and activator activity was not due to bacterial proteases. In contrast to the crevicular fluid no plasmin and activator could be demonstrated in the saliva or in samples from other areas of the oral mucose. In addition it was possible to demonstrate traces of fibrinogen in the crevicular fluid by the reaction of the latter to rabbit antibodies to human fibrinogen. The existence of an active fibrinolytic activity at the junction between the gingival epithelium and the tooth might be of significance in the etiology of periodontal diseases.