A large proportion of p-azophenylarsonate (ARS)-specific antibodies from A/J mice share a cross-reactive idiotype (CRIA) that comprises a family of closely related but nonidentical clonotypes. I determined that only 2.6 % (7 out of 267) A/J ARS-specific monoclonal antibodies generated in the splenic focus system possess the predominant CRIA. Because ARS-specific B cells are present at a frequency of 1/68,000 B cells, the frequency of the entire idiotype family is 1 per 2.8 X 10(6) splenic B cells. Thus, there is a striking discrepancy between the representation of this idiotype at the clonal precursor cell level and the serum antibody response. In addition, BALB/c mice have the potential to generate CRIA-positive precursor cells within their nonimmune repertoire. When A/J mice are immunized with ARS-protein conjugates, the serum antibody response and precursor cell population are both dominated by CRIA. The frequency of CRIA-positive B cells increases over 100-fold after immunization, whereas CRIA-negative precursor cells may initially decrease, followed by a later rise in frequency. Finally, although ARS-specific precursor cells are present in high frequency at birth, CRIA-positive monoclonal anti-ARS antibodies are not observed during the early neonatal period. These data provide evidence to suggest that complex regulatory networks influence precursor cell and serum antibody expression.