Basophil leucocyte-bound IgE has been investigated in patients with chronic urticaria by the method of reversed anaphylaxis. In this reaction basophil-bound IgE behaves as an antigen, the amount present being inversely proportional to the concentration of anti-IgE producing maximum histamine release. Dose-response curves in which histamine release was plotted against the concentration of anti-IgE failed to reveal any substantial difference in the optimum concentration of anti-IgE for maximum release from basophils of urticarial subjects compared with control subjects, thus suggesting there is no quantitative abnormality of basophil bound IgE in chronic urticaria. However, the magnitude of maximum histamine release by anti-IgE from basophils of urticarial subjects was reduced compared with controls and this was not related to serum IgE concentrations since the mean serum IgE concentration was slightly higher in the urticaria group. Studies of spontaneous and compound 48/80-evoked histamine release in the two groups did not reveal any differences in stability of histamine stores or in the biochemical histamine release mechanism over a wide range of concentrations of 48/80. These results raise the possibility of a qualitative abnormality of basophil bound IgE in chronic urticaria.