The function and several of the structural features of the C1 inactivator protein isolated from the plasma of a mother and daughter with the variant form of hereditary angioneurotic edema have been examined. These abnormal inhibitors shared immunologic identity with the normal C1 inactivator protein; however, they were inactive in inhibiting the functional activity of C1s. Analysis of the abnormal inhibitors by sodium dodecyl sulfate (SDS) acrylamide gel electrophoresis suggested that each consisted of a single polypeptide chain, the mobility of which was slower than that of the normal C1 inactivator. The apparent molecular weight of the patients' inhibitors was 109,000 daltons as contrasted to 105,000 daltons, that of the normal C1 inactivator. The abnormal inhibitors failed to form a complex with C1s or plasmin as analyzed by SDS-acrylamide gels. The large proteolytic derivatives resulting from the plasmin- and trypsin-induced degradation of the abnormal inhibitors were approximately 3,000 daltons heavier than the corresponding products derived from normal C1 inactivator. Thus, the structural abnormality identified appeared to be a property of the core molecule. Treatment of the inhibitors with neuraminidase failed to demonstrate a difference between the normal and patient-derived C1 inactivator molecule. Neither were major differences found between the amino acid composition of the defective and normal inhibitors; however, the acidic amino acids tended to be higher in the patients' inhibitors, and the phenylalanine content lower. Thus, these studies have identified both structural and functional abnormalities in the C1 inactivator protein isolated from two related patients with hereditary angioneurotic edema. Examination of the interaction between endopeptidases and the inhibitors has further delineated the abnormal structural features.