Sodium-potassium-stimulated adenosine triphosphatase and carbonic anhydrase isozymes I and II were localized immunocytochemically in adenomas, adenocarcinomas, and normal epithelium of human colon harboring non-neoplastic lesions. Non-neoplastic control colon showed carbonic anhydrase I and II in the cytoplasm of the columnar cells lining the upper half of the crypts. Antiserum to sodium-potassium-stimulated adenosine triphosphatase bound to the basolateral but not the apical plasmalemma of columnar epithelial cells. Staining was most intense in the superficial cells, which also contained carbonic anhydrase, but was also evident to a lesser degree in cells deep in the crypts. Adenomas and adenocarcinomas failed to stain for content of carbonic anhydrase but retained basolateral sodium-potassium adenosine triphosphatase positivity. The staining characteristics of colonic neoplasms for the two enzymes involved in the transport function of colonic epithelium thus resembled those of the less mature cells lining the base of normal crypts.