AbstractAn abnormal protein, antigenically identical with C1 inactivator and belonging electrophoretically to the α1‐globulins was found in plasma and serum from a patient with angioneurotic edema. Treatment of the pathological protein with 2‐mercaptoethanol or with neuraminidase followed by analysis with antigen‐antibody crossed electrophoresis showed that the protein was a complex of C1 inactivator and albumin. The patient's serum and plasma was found to contain also a C1 inactivator protein migrating with α2‐globulin but somewhat faster than normal C1 inactivator. Neither the α2‐ nor the α1‐C1 inactivator inhibited the hydrolyzing effect of C1 on N‐acethyl‐L‐tyrosine ethyl ester.