We have employed five spectrotypically distinct monoclonal anti-variable region antibodies in the definition and characterization of a set of idiotopes expressed on murine monoclonal antibodies specific for streptococcal group A carbohydrate (GAC). By evaluating which of a panel of monoclonal anti-GAC antibodies were bound by the various anti-idiotopes, we observed four distinct reactivity profiles for the five anti-idiotopes ranging from highly restricted (binding of the homologous anti-GAC monoclonal antibody only) to broadly cross-reactive (binding of 18 of the 38 IgG3 anti-GAC antibodies). With N-acetyl-D-glucosamine and soluble GAC used as haptens, this spectrum of reactivity profiles was paralleled by a gradient of susceptibility to hapten inhibition of anti-idiotope binding to idiotope. The degree of cross-reactivity exhibited by a given anti-idiotope was found to be inversely related to its susceptibility to hapten inhibition. The topographic relationships among the idiotopes, defined by the results of competitive binding assays, were suggestive of a linear idiotope map spanning the variable region from the antigen-binding site to the vicinity of the constant region. Additional data from competitive inhibition assays with isolated and recombined H and L chains from a prototype monoclonal anti-GAC antibody (HGAC 39), and from isoelectric focusing of whole or reduced and alkylated HGAC 39, suggested that one of the idiotopes was located, at least primarily, on the VL domain.