PURPOSE: Active HCO3- transport through the corneal endothelial cell layer causes a dehydration of the corneal stroma and is thought to be driven by Na/K- and HCO3(-)-dependent ATPase as well as an electro-genic Na/HCO3- cotransport. Transmembrane bicarbonate transport has also been associated with the recently characterised membrane-anchored isoform of carbonic anhydrase (CA IV) in various tissues. We investigated the localisation of CA IV in human fresh and cultured epi- and endothelium at the light- (LM) and electron-microscopic (EM) level. METHODS: Postmortem corneas were obtained within 12 hours of death, stored in formaldehyde and sectioned in paraffin. LM immunohisto-chemistry was performed using the purified gamma-globulin fraction of a polyclonal chicken antibody against CA IV isolated from human kidneys. Epi- and endothelial cell cultures were grown in uncoated flasks under standard conditions and processed both for LM and EM immunohistochemistry using the same antibody. RESULTS: Lightmicroscopy of fresh tissue showed membrane staining for CA IV in the whole circumference of the endothelium. Little staining was also observed in some cells of the basal cell layer of the epithelium. Immunohistochemical staining at the EM level was confined to the cell surface of confluent cultures of both epi- and endothelial cells. CONCLUSION: The localisation of CA IV to the cell surface of fresh and cultured corneal endothelium suggests the presence of a membrane-bound ion exchange mechanism which may be important for HCO3- transport and corneal hydration. Compromising this mechanism by treatment with local carbonicanhydrase inhibitors may be of clinical importance in selected endothelial disease.