A case of congenital afibrinogenemia is described. Despite the absolute incoagulability of afibrinogenemic blood, this type of hemorrhagic diathesis is not accompanied by hemarthrosis. Systematic studies of this patient's blood revealed that all other known clotting factors are present in normal concentration. Comparison of the electrophoretic patterns of afibrinogenemic plasma and serum confirmed the fact that except for fibrinogen no other clotting factor is detectable by routine electrophoretic analysis. This is due to the extremely small concentration of these entities when compared with the value of total serum proteins. On the basis of family studies the hereditary pattern of congenital afibrinogenemia is assumed to be recessive and not sex-linked. The rate of disappearance of fibrinogen from the blood stream after transfusion of Cohn's Fraction I has been studied. Starting from a level of 161 mg./100 cc., the zero level was reached in 12 days.