We have investigated the regulation of expression of two distinct intrastrain cross-reactive idiotypes, CRIA and CRIC , characteristic of anti-p-azophenylarsonate (anti-Ar) antibodies of the A/J and BALB/c strains, respectively, in (BALB/c X A/J)F1 ( CAF1 ) mice. Such hybrid mice were found to synthesize antibodies with each idiotype when immunized against the Ar hapten group, although the expression of each was significantly reduced as compared with the parental strain. CAF1 mice were pretreated with idiotypic-specific antibody reagents and subsequently hyperimmunized against the Ar hapten. Analysis of the idiotypes present in immune sera showed that suppression of either CRI did not concomitantly suppress the expression of the other. Alteration of the expression of one idiotype was not, however, without influence on the other; the expression of CRIC was markedly enhanced in mice suppressed for CRIA.