Résumé : The anti-ARS immune response of A/J mice is characterized by the reproducible and dominant selection of CRI(A) bearing antibodies. In this report, we have investigated the role of affinity for the antigen in the selection of antibody repertoire during an immune response. A/J anti-ARS responses with different ranges of affinities for arsonate were elicited by the injection of differently arsanylated carrier proteins. The selection of higher affinity A/J anti-ARS responses was shown to be associated with the induction of higher levels of CRI(A) bearing anti-ARS antibodies. A detailed idiotopic analysis also showed a more precocious selection of the CRI(A) 'canonical combination' in the higher affinity anti-ARS responses. These results strongly suggest an important role for affinity and clonal selection in the dominant expression of the CRI(A) idiotype in the A/J anti-ARS response.