Hereditary Antithrombin III Deficiency in an English Family

S ummary . A family including four members with decreased antithrombin III (ATIII) measured by a biological and an immunological method is described. Immunologically measured levels of other coagulation inhibitors were normal. In the antithrombin III deficient family members there was a history of thrombotic disease. Turnover of 125 I‐antithrombin III in a deficient individual showed the lower plasma levels probably to be due to a deficiency in the rate of synthesis rather than to an increased catabolic rate.