Using monoclonal antiidiotypic antibodies, we developed a sensitive binding assay that detects molecules with one or with two idiotopes of the ABPC48 idiotype. ABPC48 cross-reactive idiotypes were thus shown to be present in substantial amounts in sera of nonimmunized mice. Levan binding sites are found on these idiotypes. During the life time of the mice, the natural anti-levan titer increases while ABPC48 idiotypic expression remains constant, suggesting different controls for these two activities. On the other hand, ABPC48 cross-reactive idiotypes participate--as minor components--in the response that follows a deliberate immunization with bacterial levan. This induction process is likely to reflect the selection of idiotopes expressed by the B cell clones preactivated in sera of nonimmunized mice rather than the activation of silent clones. We suggest that a similar situation might explain the reported emergence of ABPC48 idiotypes in animals primed with antiidiotypic antibodies and subsequently stimulated with levan.