Circulating histamine releasing factor(s) have been demonstrated previously in chronic urticaria by an immediate weal-and-flare response to intradermal autologous serum injection. We have studied 25 chronic urticaria patients by in vivo skin testing with autologous sera and an in vitro histamine release assay using mixed leukocytes of healthy donors, to define the nature and functional properties of the serum factor(s). Twenty showed a weal response to autologous serum (mean ± s.e.m., 37.3±6.8 mm3). Weal formation was confined to ultrafiltered serum fractions > 100 kD in nine of nine patients. There was no response in 10 healthy controls or five patients with symptomatic dermographism. Fourteen chronic urticaria sera elicited histamine release > 10% (mean ± s.e.m., 44-3%± 6 7) above basal levels from leukocytes of at least one of seven healthy donors. This in vitro response was also confined to ultrafiltered serum fractions > 100 kD in seven of seven sera and was present in IgG fractions of six of seven chronic urticaria sera that showed histamine releasing activity. Functional studies indicated that this histamine releasing autoantibody had the properties of anti-IgE: chronic urticaria sera ‘desensitized’ basophil leukocytes to subsequent challenge with other chronic urticaria sera and to goat anti-human IgE antibody; human myeloma IgE inhibited histamine release from leukocytes in response to chronic urticaria sera; removal of surface-bound IgE by lactic acid “stripping” reduced histamine release in response to chronic urticaria sera and anti-IgE and subsequent passive sensitization with IgE myeloma serum partially restored it, Stainable peripheral blood basophils/mm3 in chronic urticaria patients were significantly reduced (mean ± s.e.m., 7.9 ± 2 0) when compared to healthy controls (39.6 ± 44), P < 0.001. These results suggest that histamine releasing autoantibodies are important in the pathogenesis of chronic urticaria by stimulating or facilitating degranulation of basophils and cutaneous mast cells through cross-linking cell surface IgE receptors.