A family with a new congenital abnormality of antithrombin III (AT III) is presented. 5 members, all females, were affected. The proposita has had several thrombotic manifestations. The other patients, so far, are asymptomatic. Antithrombin activities were all decreased regardless of the method used (chromogenic or clotting) and regardless of the presence or absence of heparin in the assay system. AT III antigen, on the contrary, was normal in all patients regardless of the method used (electroimmunoassay, radial immunodiffusion or Laser nephelometer). The crossed immunoelectrophoresis without heparin showed in plasma the presence of an abnormal peak which was more anodal than the normal counterpart. The same pattern was seen in serum. In the heparin-modified cross-immunoelectrophoresis a normal pattern was seen in plasma and an abnormal one in serum. In the latter the anodal peak was in fact larger than the normal counterpart. Chromatographic studies using Heparin-Sepharose column failed to show changes in heparin affinity, and indicated that both the normal and the abnormal proposed to describe this abnormality. These studies further emphasize the great heterogeneity of AT III defects. This is the first AT III abnormality to show an abnormal crossed-immunoelectrophoresis in the absence of heparin.