We have generated high titers (up to 1 mg/ml) of antibodies to isotypic determinants of IgE by immunization of A/J mice with syngeneic monoclonal IgE conjugated to keyhole limpet hemocyanin. As much as 3 mg of anti-idiotypic antibodies per ml was induced at the same time. In contrast to conventional rheumatoid factors, the anti-isotypic antibodies are of moderately high affinity (10(7)-10(8) M-1). Assays of the anti-IgE antisera indicated the presence of IgE, both free and in the form of immune complexes; the latter values are minimum estimates owing to masking of isotypic determinants. Regulatory effects of these high titers of anti-IgE can now be investigated. Such studies will be facilitated by the availability of monoclonal, syngeneic anti-IgE antibodies.