Antisera were raised in rabbits against two mouse monoclonal anti-porcine growth hormone (pGH) antibodies. Anti-idiotypic antibodies were isolated from rabbit sera by successive passage over immunoadsorbent columns of normal mouse 1g (mIg), followed by the specific immunizing monoclonal and elution from the latter. Enzyme linked immunoadsorbent assay (ELISA) for anti-idiotype and free idiotype were established. While showing specificity for their respective inducing monoclonals, anti-idiotypes also cross reacted in varying degrees with other anti-pGH monoclonals regardless of specificity differences between the antibodies, demonstrating the presence of a cross-reactive idiotype.