Polyclonal antibodies were produced against rat kidney gamma-glutamyl transpeptidase (GGT) and against a synthetic peptide corresponding to residues 512-534 in rat GGT. The anti-peptide antibody bound denatured GGT, acivicin-treated GGT and GGT absorbed to microtiter plates, but not GGT in solution, and did not inhibit GGT. The antibody against native GGT inhibited its activity in solution and did not bind efficiently adsorbed GGT in direct ELISA. It was active in direct and competition ELISA using kidney brush border membranes as the adsorbed antigen. The results indicate that these antibodies recognize conformational rather than sequence epitopes in GGT, and that marked changes occur in the conformation of GGT upon its absorption to plates or its reaction with acivicin.