The intracellular distribution of carbonic anhydrase was studied in nine human donor kidneys by the histochemical method of Hansson and by an immunofluorescence technique using antisera specific against the cytoplasmic isoenzymes HCA B and HCA C. Only HCA C was found in the renal tubules. Convoluted proximal tubules showed enzyme staining at the brush border and the basolateral membranes, and in the cytoplasm. No staining was observed in straight proximal tubules. The initial part of the thin limb of Henle's loop displayed cytoplasmic staining, whereas the distal part was unstained in most nephrons. In the medullary ascending thick limb enzyme was present at apical and basolateral cell membranes as well as in the cytoplasm, whereas in the apical cell region of the cortical ascending thick limb there was no distinct staining. The cells of the macula densa showed staining only at the cell membranes. The distal convoluted tubule exhibited heavy staining in the cytoplasm and at the cell membranes. In the initial collecting tubule and the collecting duct very intensely stained intercalated cells were found among less strongly stained chief cells. In the inner medullary segment of the collecting duct, the staining for carbonic anhydrase gradually disappeared. Many peritubular capillaries showed enzyme staining, while glomerular capillaries and larger vessels were negative. Specific fluorescence for HCA B was observed in the capillary walls.