Background: The specificity of naturally occurring IgG-subclass anti-IgE autoantibodies (a-IgE Ab) was studied by ELISA inhibition assay with a highly purified IgE-DES myeloma protein as a solid-phase antigen. Because the IgG isotypes differ in effector functions, detailed specificity studies of IgG subclass anti-IgE antibodies might clarify their putative role. Methods: Selected sera containing a high concentration of a single IgG a-IgE Ab subclass were allowed to react with purified immunoglobulins of all five classes including five different IgE myeloma proteins and papain-derived Fabϵ and Fcϵ fragments of IgE-DES. Results: The inhibition results indicated that IgG1, IgG2, and IgG3 a-IgE Ab reacted in a low-affinity reaction with IgE myeloma protein–restricted determinants because only IgE-DES was inhibitory. Moreover, the reacting epitopes were heat resistant (2 hours, 56° C) and localized in the Fabϵ-DES fragment. In sharp contrast, IgG4 a-IgE Ab reacted with high affinity to all five IgE myeloma proteins involving heat-susceptible epitopes confined to the Fcϵ fragment. Conclusions: It seems that a majority of IgG a-IgE Ab have a specificity for nonisotypic myeloma-restricted determinants whereas IgG4 a-IgE Ab were mainly isotype specific. These findings ought to be taken into account in the consideration of physiologic implications of IgG a-IgE Ab and in the interpretation of previous investigations in which intact IgE myeloma proteins have been used to detect IgG a-IgE Ab. (J ALLERGY CLIN IMMUNOL 1995;95:1011-9.)