Haemostatic plug formation in four patients with severe haemophilia A (VIII:C less than 1%) was studied in skin biopsies taken at 3, 10 and 30 min and 2 h after a template bleeding time wound had been made. The primary haemostatic plug showed relatively minor changes, consisting of a delay in platelet degranulation and interdigitation. Some platelet aggregates not attached to vessels were encountered in the wound. Subsequently the primary haemostatic plug changed into a firm stable degranulated mass of interdigitated platelets. The major abnormality occurred during the fibrinous transformation. At 2 h many haemostatic plugs consisted of a thin peripheral layer of fibrin and platelet remnants around a central area containing red and white blood cells with a varying amount of plasma and only relatively few fibrin fibres. These observations suggest that fibrin formation in the periphery of the plug is less dependent of factor VIII than in central areas. The lack of fibrin formation in the centre of the plug compensating for the platelet lysis at 2 h may have caused the central erosion of the plug.