Serum immunoglobulins, complement and α1-antitrypsin were assayed in forty-eight patients with chronic urticaria. Thirteen cases had chronic cold urticaria and thirty-two had chronic idiopathic urticaria. Elevated mean serum IgM was found in chronic cold urticaria. Seven patients had partial immunoglobulin deficiencies. IgE was elevated in sixteen cases of chronic idiopathic and in two with chronic cold urticaria. Eight patients had depressed serum total haemolytic complement activity. Low C3 and normal C4 serum protein concentrations in four cases suggested alternative complement pathway activation. Twenty of forty-six patients were atopic, although specific allergies responsible for the urticaria were not identified in any of them. α1-antitrypsin levels were normal in all patients. The data suggest that the aetiology and pathogenesis of chronic urticarias in this study are heterogeneous. No evidence of abnormality of the protease inhibitor system in either chronic idiopathic or chronic cold urticaria was found.