Thrombin or factor Xa added to plasma are inactivated by antithrombin III (At-III). The inactivation is accelerated by heparin, permitting assay systems which rapidly measure the At-III content of diluted plasma. Without heparin, the slow inactivation rates may be measured. Existing activity assays (fibrinogen or chromogenic substrates) and immunoassays of At-III have been reviewed. Correlation studies show a close correlation between the results of immunoassay and the results of most activity assays. In health, a narrow range of At-III has been found. The level is low in infancy. Fertile women have on the average somewhat lower levels than men. In old age, the level tends to drop. In clinical material studied with amidolytic assays, subnormal At-III levels were found in hereditary deficiency, liver disease, disseminated intravascular coagulation and in some cases with acute thrombosis. The amidolytic assays are rapid to perform, do not require experience in clotting technique and seem preferable in clinical routine work.