Anti-idiotypic antibody raised against the BALB/c myeloma proteins TEPC-15 or HOPC-8 inhibits induction of the immune response to phosphorylcholine (PC). (Fab′)2 and Fab′ fragments from anti-idiotype serum were prepared and anti-idiotypic antibody was purified by absorption and elution from idiotype or anti-IgG subclass immunoabsorbents. The parent material, the purified anti-idiotypic antibodies and the Fab′ fragments were assayed for: i) binding to the idiotype by using a sensitive radioimmunoassay, and ii) specific suppression of the response to PC in vitro. The correlation of binding to suppression was very similar for anti-idiotype serum and both kinds of purified antibody. However, the Fab′ fragments only bound to the idiotype and did not suppress. These findings indicate that the Fc portion of anti-idiotypic anti-receptor antibody (ARA) is essential for inducing suppression but that ARA-induced suppression is independent of the IgG isotype. It is postulated that idiotype suppression is mediated by the interaction of anti-idiotype with the antigen receptor and the Fc receptor thereby cross-linking both receptors.