Antithrombin III (AT-III) was studied in a thrombophilic family with an abnormal AT-III molecule (antithrombin III Budapest) using a modified crossed immunoelectrophoresis technique, gel filtration, 'rocket' immunoelectrophoresis and a heparin cofactor assay. When agarose was applied in the first phase of the crossed immunoelectrophoresis, the normal and the pathological AT-III revealed identical electrophoretic mobility. However, when heparin was mixed with agarose in the first phase of electrophoresis, the propositus' plasma displayed a different AT-III pattern from normal plasma. His plasma contained the first component of the normal plasma (Immune Antithrombin III1, IAT-III) in a concentration of only 5% of normal, and a protein in high concentration which although immunoreactive to AT-III antisera, had an electrophoretic mobility similar (but not identical to that of IAT-III2. This abnormal protein had no heparin cofactor activity and a molecular size greater than normal plasma AT-III. Unlike AT-III, the addition of heparin did not change the molecular size of the pathogic AT-III molecule significantly. The abnormal protein was present in lower concentrations in the patient's children and at the time of study they had no clinical or laboratory evidence of intravascular coagulation.