Preinjection of C57BL/6 mice with nano-to microgram amounts of a monoclonal IgG1 antibody directed against a binding site-related idiotope of the anti-NP [(4-hydroxy-3-nitro-phenyl)acetyl] antibody B1-8 results in enhancement or suppression of the corresponding and of another B1-8 idiotope in a subsequent anti-NP response, depending on the dose of the injected anti-idiotope antibody. The enhancing and suppressive effects appear two weeks after anti-idiotope administration and are maximal after 6-8 weeks. They are predominantly expressed at the level of IgG, not IgM, antibodies. Enhancement of idiotype expression, i.e. idiotypic memory, can also be induced by the injection of idiotypic antibody of the IgM class, namely antibody B1-8. This effect might represent one of the general mechanisms by which immunological memory is established.