This report describes two patients with haemophilia A who developed a transient thrombocytopathy with haemorrhagic diathesis during post-operative high-dose replacement therapy with antihaemophilic globulin. At the time of the bleeding the factor VIII-activity was in the normal range in both patients. The fibrinogen level, however, was elevated to 1700 mg-% and the factor VIII-associated antigen rose to more than 6-fold. At no time of replacement therapy with antihaemophilic globulin could either fibrinogen split products or fragments of the factor VIII-protein be detected by the usual methods. In view of the results of the thrombocyte aggregation experiments the authors postulate a disturbance of platelet function at the level of the membrane surface due to an overload of increased amounts of circulating proteins. Both the possible interference of dialysable factor VIII-components and the role of immunpathologic phenomena are discussed.