gamma-Glutamyltranspeptidase (gamma-GT) was localized in slices of rat lung, at the ultrastructural level, by pre-embedding immunogold labelling. Antiserum was raised against the protein purified from rat kidney. The enzyme was found to be concentrated on the lumenal surface of the non-ciliated ("Clara") cells of the bronchiolar epithelium and, to a lesser degree, on the surface of type II alveolar pneumocytes. This immunological localization was consistent with the distribution of reaction product, in both slices and resin sections incubated to demonstrate gamma-GT activity. gamma-GT is probably involved in the utilization of reduced glutathione (GSH) present in the fluid lining the airway epithelium.