Abstract. Quantitative estimations of serum IgG, IgA and IgM were performed in 30 individuals with congenital coagulation defects and in 47 related persons.The majority of patients with hemophilia, congenital defects of the prothrombin complex and congenital afibrinogenemia showed a significant elevation in one or more immunoglobulins. By contrast, normal immunoglobulin concentrations were recorded in 3 persons with yon Willebrand's disease, in 2 normal subjects with isolated factor VIII deficiency, and in 1 female patient with acquired deficiency of the liver-dependent clotting factors.In 11 out of 15 mothers of hemophiliacs immunoglobulin concentrations were significantly elevated. A high proportion of other relatives of these patients likewise showed significantly raised immunoglobulin concentrations. Follow-up tests on patients and their relatives over more than 1 year indicated that findings usually did not vary appreciably.On the basis of our study it is suggested that in patients with congenital coagulation defects elevated immunoglobulin concentrations are hereditary.