Permin and Astrup used a plate method of demonstrating fibrinolytic activity1,2. This technique can be applied histologically. A thin layer of fibrin is made on a microslide by adding a drop or two of fibrinogen to a film of thrombin on the glass. When the fibrin layer has formed, a section of fresh or alcohol-fixed tissue cut on the freezing microtome is applied to its surface and the preparation incubated at 37° C. After 30-60 min. patches of digestion of the fibrin appear in relation to structures possessing fibrinolytic activity. These patches appear as clear zones when the preparations are stained with hæmalum and eosin (Fig. 1), and can be related to the structures in the section.