This paper describes the development of methods for quantitation of an intrastrain cross-reactive idiotype (CRIA) in IgE antibodies directed to the p-azophenylarsonate (Ar) group and their application to normally immunized and idiotypically suppressed mice. The data were validated by the use of monoclonal and serum anti-Ar antibodies known to possess or lack CRIA. The idiotype was absent in the IgE as well as the total anti-Ar population of idiotypically suppressed mice. A good correlation was noted between the CRIA content of total and IgE anti-Ar with a trend toward a somewhat lower content in the IgE population. Irradiated mice that received immune cells prior to further immunization expressed relatively low concentrations of the idiotype (per weight unit of anti-Ar). The method will be useful in studies of the regulation of the switch to IgE synthesis.