Recent studies have shown that monoclonal anti-alprenolol antibody 37A4 could mimic the beta-adrenergic receptor with respect to binding properties and alprenolol-induced changes in intrinsic fluorescence. We prepared and characterized polyclonal antibodies against a synthetic tetradecapeptide corresponding to residues 92-105 of the 37A4 VH domain. In ELISA and immunoblotting assays, the anti-peptide antibodies bound to the intact IgG molecule as well as to the isolated heavy chain. They failed to interact with the native IgG protein but recognized with high specificity the conformationally modified Ig molecule; as a consequence, their reactivity to the entire protein was found to vary with the conditions of the assay. This study illustrates that anti-peptide antibodies might be used as probes for the detection of conformational changes occurring in the Ig hypervariable regions.