A molecular antithrombin III variant (Antithrombin III Roma) with an abnormal pattern of crossed immunoelectrofocusing was further investigated in order to identify the pathological isoforms. AT III crossed immunoelectrofocusing of the whole plasma from the affected patients showed a loop overlapping the peak normally present at pH 4.8-4.6. Affinity chromatography demonstrated the presence of an AT III fraction totally lacking in heparin affinity. Crossed immunoelectrophoresis on heparin-agarose (H-CIE) and crossed immunoelectrofocusing (CIEF) runs performed on the fractions obtained by heparin-agarose affinity chromatography confirmed that the functional defect was exclusively related to the pathological isoantithrombin (pH 4.8-4.6), which was also devoid of any progressive activity. The AT III fraction with normal affinity to heparin displayed H-CIE and CIEF patterns identical to the control AT III.