The anti-IgE autoantibody in the IgG class was detected in 95.5% of patients with atopic asthma, and in 72.2% of those with non-atopic asthma, using a newly established solid phase enzyme immunoassay (EIA). The specificity of anti-IgE autoantibody was confirmed by both competitive inhibition and absorption experiments, using human IgG, IgA, IgM, IgD, IgE and rabbit anti-human IgG. Significant correlations were observed between the levels of the anti-IgE autoantibody and the serum IgE. Gel filtration studies indicated that the anti-IgE autoantibody in sera from asthmatic patients was present in the immune complex form with self-IgE, in addition to the monomeric antibody. Furthermore, this anti-IgE autoantibody has the ability to induce the reversed type immediate skin reaction, in asthma free individuals. These observations strongly suggest the putative role of the anti-IgE autoantibody in the modulation of IgE-mediated immune systems and the pathogenesis of bronchial asthma.