Rabbit reagents previously thought to display specificity for a cross-reactive idiotype on anti-VHa allotype antibody from all tested rabbits have recently been shown to be contaminated with an induced (latent) molecule similar or identical to the original antigen (rabbit a1 or a2 allotype). In an attempt to circumvent this problem, we have immunized guinea pigs with rabbit anti-a1 allotype antibody to produce heterologous anti-idiotype antibody. The resulting guinea pig antibody (GP anti-R IdX) recognizes anti-a1 antibody from each of 17 immunized rabbits, and in four tested samples reacts with 22 to 100% of the molecules. Neither goat nor guinea pig anti-a1 reacts with the guinea pig anti-R IdX antibody, even though the goat, guinea pig, and rabbit anti-a1 Ab all recognize a similar set of a1 determinants. The reaction between IdX-bearing rabbit anti-a1 and guinea pig anti-R IdX is inhibited by the original antigen (a1 IgG), demonstrating that the IdX is at or near the antigen combining site of anti-a1 antibody. Immunoelectron microscopy of immune complexes supports this conclusion and demonstrates that the reactive site on the GP anti-R IdX is at or near its antigen combining site.