THE fibrin stabilizing factor (FSF) is a thermolabile plasma protein which in the presence of calcium converts a loosely cross-linked, urea-soluble fibrin clot (fibrin s) into a urea-insoluble gel (fibrin i)1,2. FSF reacts sto-chiometrically with fibrinogen in the latter's conversion to fibrin3 and requires the presence of free sulphydryl groups for its activity4. The recently established enzymatic nature of this factor has led to the suggestion that it be renamed ‘fibrinase’5. Differences in the activity of plasma FSF have been found to be associated with a variety of clinical conditions6. More recently, the occurrence of congenital deficiencies of FSF has been reported and shown to be responsible for abnormal haemostasis and poor wound healing7,8. In this connexion, it is of interest that fibrin s clots do not support the growth of fibroblast cultures nearly so well as normal plasma clots9, a phenomenon which may in part, at least, be due to the fact that they are less resistant to lysis by plasmin10.