BALB/c mice were immunized with purified phosphorylcholine (PC)-specific myeloma proteins of the TEPC-15 and M167 tumors. After the respective isologous anti-idiotypic responses had been established, and after an additional resting period of four months, animals were immunized with phosphorylcholine-hemocyanin in a procedure normally leading to formation of IgE in addition to antibodies of other classes. The experiment shows that BALB/c mice producing (T15) anti-idiotypes depress the formation of anti-PC IgE, IgM, IgA antibodies, while stimulatory effects on the IgG response are observed. This demonstrates that IgE bears idiotypes similar or identical, to antibodies of other classes, and that anti-PC-IgE antibodies mainly bear the T15 idiotype or idiotypes crossreacting with isologous anti-T15. On the other hand, cells responsible for regulation or production of antibodies of different classes show different susceptibility to regulation by anti-idiotypes. The formation of IgE antibodies seems to be particularly susceptible to suppression by anti-idiotypes, a fact which may lead to new treatments for some allergic diseases and to new insight into the mechanisms of hyposensitization therapy.