HMWK and LMWK were purified from human plasma by ion exchange and affinity chromatography and used to raise anti-kininogen monoclonal antibodies. The Ig class and affinity of these Mab's that are directed at the heavy chain of the kininogens, were determined. Plasma samples of two patients, one with acquired C1-inhibitor deficiency and angioedema and one with idiopathic angioedema were analized by immunoblotting with these Mab's. Both were found to have cleaved HMWK during clinical attacks.