An abnormal AT III variant was found in five members from a family where a high incidence of thromboembolism occurred. In all the affected subjects AT III antigen concentration was normal, whereas antithrombin and antifactor Xa progressive activities as well as heparin cofactor activities were low. Crossed immunoelectrophoresis performed either in absence or in presence of heparin showed a normal plasma pattern. Further chromatographic investigations showed a normal affinity to heparin. An abnormal plasma pattern was evidentiated by crossed immunoelectrofocusing throughout all the AT III pH range. These data are consistent with the presence of an abnormal AT III variant with a defective binding to serine proteases and clearly identifiable only by crossed immunoelectrofocusing. This variant appeared different from the other qualitative AT III defects so far described and was named 'Antithrombin III Pescara'.