AbstractAnti‐p‐azophenylarsonate (anti‐Ar) antibodies arising in strain A mice immunized with keyhole limpet hemocyanin‐Ar have previously been shown to exhibit strong idiotypic cross‐reactions. The appearance of the cross‐reacting idiotype is suppressed by prior administration of antiidiotypic antibody, but a high titer of anti‐Ar antibodies lacking the idiotype is produced in response to the antigen. The present studies indicate that these antibodies do not share idiotype with anti‐Ar antibodies arising in other suppressed or nonsuppressed mice of the same strain. Strain A mice therefore produce at least 2 populations of anti‐Ar antibodies, one showing intrastrain cross‐reactions and the other being unique to each mouse. The results emphasize the great diversity of anti‐hapten antibodies produced within an inbred strain.Suppression persisted when 9 weeks were permitted to elapse between administration of antiidiotypic antibody and the first challenge with antigen. This gives further evidence that suppression takes place at a central level and not through simple neutralization by antiidiotypic antibody of anti‐Ar antibodies bearing the cross‐reactive idiotype. Suppression of idiotype is maintained for a t