Since a protease inhibitor or anaphylatoxin inactivator deficiency might explain why certain individuals are prone to develop chronic urticaria/angioedema or anaphylactoid reactions to radiographic contrast media, serum alpha 1-protease inhibitor, alpha 1-antichymotrypsin, alpha 2-macroglobulin, inter-alpha-inhibitor, antithrombin III, alpha 2-plasmin inhibitor, C1 inhibitor, and serum carboxypeptidase N were assessed by immunologic or functional methods. These values all were within normal limits in both groups of patients except for a low mean alpha 1-protease inhibitor level in chronic idiopathic urticaria/angioedema and cold urticaria patients and marginal decreases of alpha 1-protease and inter-alpha-inhibitor levels in radiographic contrast medium reactors. However, these abnormalities were not thought to be of pathogenetic significance.