The immune response to the phosphorylcholine (PC) hapten elicited by PC-keyhole limpet hemocyanin (KLH) is composed of 2 groups of antibodies with specificity to either PC or phenylphosphorylcholine which were designated as group I and II anti-PC antibodies, respectively. We demonstrate that anti-PC IgE antibody expression is restricted to group II antibodies and does not display the T15 idiotype. Accordingly anti-PC IgE antibodies recognize the same epitope (PC-phenyl) as IgG1, IgG2a and IgG2b antibodies which is different from that (PC) recognized by IgM and IgG3 antibodies. A monoclonal anti-PC IgE antibody, representing group II characteristics was established. From amino acid sequences of light chains of purified group I and II antibodies from serum as well as of monoclonal representatives thereof it appears that both populations are relatively homogenous and represent independent clonal expressions. Nevertheless the formation of anti-PC IgE antibodies in mice can be suppressed by isologous anti-T15 anti-idiotypic antiserum. This antiserum, however, crossreacts with different anti-PC antibodies including monoclonal group II anti-PC IgE antibodies and is composed of a large number of anti-idiotopes. An analyses performed with monoclonal anti-T15 idiotopes demonstrates that some but not all antibodies suppress the formation of anti-PC IgE. We conclude that syngeneically induced anti-idiotypic interactions may display regulation of a wide range of specificities affecting responses to various antigenic determinants.