The strain A anti-p-azophenylarsonate major cross-reactive idiotypic family includes members with no reactivity toward p-azophenylarsonate.

The sera of immunized A/J mice contain low but detectable levels of immunoglobulin, bearing previously described cross-reactive idiotypic (CRI) determinants diagnostic of the strain A anti-p-azophenylarsonate (Ars) response. Such molecules from nonimmune sera cannot be adsorbed onto affinity columns coupled to a high density with Ars. After extensive immunization with Ars-coupled proteins, Ars-nonbinding CRI is found at the same low level, even though substantial Ars-binding CRI appears. On the other hand, immunization with a monoclonal rat anti-CRI, which was originally raised against Ars-binding CRI, elicits high concentrations of Ars-binding as well as Ars-nonbinding CRI immunoglobulin. Three hybridoma proteins produced from an A/J mouse immunized with the rat anti-CRI react with all tested anti-idiotypic sera from three species of animals, but show no reactivity toward Ars in several different assays. One hybridoma protein from the same fusion demonstrates Ars-binding capacity.