S U M M A R YMembrane-associated carbonic anhydrase (CA) has a crucial role in renal HCO 3 Ϫ absorption. CA activity has been localized to both luminal and basolateral membranes of the tubule epithelial cells. CA XII is a transmembrane isoenzyme that has been demonstrated in the basolateral plasma membrane of human renal, intestinal, and reproductive epithelia. The present study was designed to demonstrate the distribution of CA XII expression in the rodent kidney. A new polyclonal antibody to recombinant mouse CA XII was used in both Western blotting and immunohistochemistry. Western blotting analysis revealed a 40-45-kD polypeptide in CA XII-expressing CHO cells and isolated membranes of mouse and rat kidney. Immunofluorescence staining localized CA XII in the basolateral plasma membranes of S1 and S2 proximal tubule segments. Abundant basolateral staining of CA XII was seen in a subpopulation of cells in both cortical and medullary collecting ducts. Double immunofluorescence staining identified these cells as H ϩ -secreting type A intercalated cells. The localization of CA XII in the peritubular space of proximal tubules suggests that it may play a role in renal HCO 3 Ϫ absorption, whereas the function of CA XII in the type A intercalated cells needs further investigation.