Previous reports have shown the A/J anti-para-azophenylarsonate (anti-Ar) antibodies that share a major cross-reactive idiotype (CRI) comprise a family of closely related but nonidentical molecules. Serological studies with CRI+ monoclonal anti-Ar antibodies have suggested the presence of a conserved idiotypic determinant within the family. The present study utilized monoclonal ant-idiotypic determinant within the family. The present study utilized monoclonal anti-idiotypic antibodies to define further the nature of the conserved idiotypic determinant. It was found that 8 of 10 CRI+ monoclonal antibodies possess an idiotypic determinant reactive with each of three monoclonal anti-idiotypic antibodies. In addition, approximately 60% of CRI+ serum anti-Ar antibodies reacted with one of the monoclonal anti-idiotypic preparations. The monoclonal anti-idiotypic antibodies react with an idiotope in the region of the hapten-binding site, as indicated by the ability of free haptens to inhibit idiotype-anti-idiotype interactions. Finally, two of three monoclonal anti-idiotypic antibodies suppressed the subsequent production of CRE+ serum anti-Ar antibodies when administered before antigen, without significantly affecting the total anti-Ar response.