Specific anti-a1 Ab was isolated from rabbits, guinea pigs, mice, chickens, and a goat. Each of these preparations was able to inhibit the reaction between a1 IgG and rabbit anti-a1 Ab as well as the reaction between rabbit IdX (anti-a1 Ab) and rabbit anti-IdX (anti-anti-a1 Ab). Maximal levels of inhibition ranged from 75 to 100% in the latter assay. Although the relative binding efficiencies of each preparation varied widely, there was generally a positive correlation between the ability of an anti-allotype reagent to bind to a1 IgG and to anti-IdX Ab. Each of the heterologous anti-a1 Ab samples was able to form precipitin bands with rabbit anti-Id Ab. These bands fused with each other and with rabbit anti-a1 Ab. These results weaken the interpretation that the ubiquitous expression of IdX previously observed in the rabbit reflects shared conserved genes. We suggest that either 1) the anti-Id Ab represents high fidelity internal images of the a1 epitopes, or 2) latent a1 allotype Ig is present in the anti-IdX preparation. In either case, any anti-a1 Ab would be potentially reactive with the anti-IdX preparation.