C 1-inhibitor (C1-Inh) is an important inhibitor of the inflammatory response and deficiency of this inhibitor, which may be hereditary or acquired, is associated with recurrent episodes of edema. Recently, an autoimmune form of angioedema has been described that is associated with functional deficiency of C1-Inh and an autoantibody that impedes C1-Inh function. In this report we describe the isolation of C1-Inh from the monocytes and plasma of a patient with autoimmune angioedema and demonstrate that the patient's monocytes secrete structurally and functionally normal C1-Inh, but show that this protein circulates in the patient's plasma in an inactive, structurally altered form. Furthermore, using analytic gel electrophoresis techniques it is demonstrated that the patient's autoantibody facilitates cleavage of normal C1-Inh, by its target proteases, to the same species of C1-Inh that is found circulating in the patient's plasma. This autoantibody facilitated cleavage of normal C1-Inh is apparently a consequence of destabilization of protease/inhibitor complexes. These findings contribute to our understanding of protease/C1-Inh interactions and document important observations on pathogenic mechanisms in autoimmune disease.