Summary. Immunoelectrophoretic studies were carried out on plasma from a patient with an abnormal antithrombin III molecule (antithrombin III ‘Budapest’). One-dimensional immunoelectrophoresis using a commercial antibody to antithrombin III (antibody to α2,-AtIII) showed two precipitin peaks of α2-AtIII antigen. Two-dimensional immunoelectrophoresis showed two precipitin peaks, one with normal electrophoretic mobility, the other with increased electrophoretic mobility. It was shown that α2-AtIII antigen with normal mobility was identical to normal α2-AtIII, and that the α2-AtIII antigen with increased electrophoretic mobility was antigenically deficient, and appeared to be present in higher concentration than the normal α2-AtIII antigen. Although one-dimensional immunoelectrophoresis on plasma from the patient's son showed only one peak, two-dimensional immunoelectrophoresis revealed that the son also had two populations of α2-AtIII, one with normal mobility, the other with increased mobility. However, the α2-AtIII antigen with normal mobility appeared to be present in higher concentration than the α2-AtIII antigen with increased mobility. One- and two-dimensional immunoelectrophoresis using a different commercial antibody to α2-AtIII showed only one precipitin peak using the patient's plasma. The precipitin peak observed following two-dimensional immunoelectrophoresis was asymmetric and showed increased mobility.