Rabbits were inoculated with affinity-purified human antibodies to the common house dust mite Dermatophagoides pteronyssinus (DPT) to produce anti-idiotypic (Id) antibodies. The Id carried by human IgE antibodies were studied in a radioimmunoassay in which specific anti-DPT IgE were first insolubilized on rabbit F(ab')2 fragments against human Fc epsilon. In a single individual specific anti-DPT IgE antibodies shared bystander idiotopes (i.e. idiotopes located outside the antigen-binding site) with IgG antibodies of the same specificity, since the adsorption on insolubilized anti-DPT IgG almost completely eliminated the binding of anti-Id antibodies to IgE. This differed from our previous observations (J.-M. R. Saint-Remy et al., Eur. J. Immunol. 1986. 16: 575) wherein distinct paratope-associated idiotopes were identified on IgG and IgE anti-DPT antibodies. The frequency of cross-reactions between idiotopes of anti-DPT IgE antibodies observed in ten unrelated individuals was less than 10% for bystander idiotopes. This finding confirmed the private character of the above idiotopes that we found earlier on anti-DPT IgG antibodies. However, one anti-Id antibody preparation was found to react with bystander idiotopes of anti-DPT IgE of most individuals, thereby suggesting the existence of a recurrent idiotope. The low frequency of inter-individual bystander idiotope cross-reactivity contrasted with the public character of paratope-associated idiotopes.