Inoculation of syngeneic IgE into 2- to 12-day-old mice results in prolonged synthesis of anti-IgE antibodies without further challenge. These anti-IgE antibodies may be largely responsible for the long-term inhibition of synthesis of IgE that is known to result from a perinatal challenge with IgE. This conclusion is supported by the effect of passive inoculation of syngeneic polyclonal anti-IgE antibodies into young mice, which similarly results in selective inhibition of IgE synthesis. Further evidence is the close relationship between the age dependency of IgE-induced inhibition of subsequent IgE synthesis and the ability of IgE to induce anti-IgE antibodies. IgE synthesis was monitored at the level of secretion by B cells as well as serum IgE levels and IgE antibody responses.