Circulating histamine-releasing factors have been identified in the serum and plasma of chronic-urticaria patients by in vivo skin testing and in vitro histamine release from heterologous mixed leukocytes. Quantitative mast cell studies of serum skin test biopsies and electron microscopy indicate that the serum factors release histamine by mast cell degranulation. Peripheral blood basophils and total cellular blood histamine are reduced in chronic-urticaria patients suggesting that the circulating serum factors cause sustained degranulation. Histamine-releasing activity has been identified by skin testing in ultrafiltered serum fractions 100 kDa and was present in IgG purified from some chronicurticaria sera by protein G affinity chromatography. The dose-response relationship and kinetics of histamine release in vitro were similar to those of anti-human IgE. ‘Desensitisation’ of basophils by prior incubation with anti-IgE in the absence of calcium and competitive inhibition studies with myeloma IgE serum indicated that histamine-releasing autoantibodies in chronic-urticaria sera and purified IgG have the properties of anti-IgE. Plasma exchange in 4 patients with active chronic urticaria refractory to antihistamine therapy showing in vivo and in vitro histamine-releasing activity was followed by temporary remission of disease activity in 2 of them. It is possible that chronic urticaria is an autoimmune dise