Cultured monolayers of dog kidney (MDCK) cells display many features of in vivo epithelia. This work describes the identification of two separate strains of MDCK cell with entirely different properties. Strain I cells form epithelial monolayers which display a high electrical resistance (4.1 k omega . cm-2); the basal short-circuit is small (approx. 0.5 muamp . cm-2) and is stimulated by adrenaline (1 micrometer) prostaglandin E1 (1 micrometer) and arginine vasopressin (2 micrometer) added to the basal bathing solution. Strain II cells form epithelial monolayers of low electrical resistance; the short circuit current is insensitive to adrenaline, prostaglandin E1 and vasopressin. Strain II cells possess measurable activities of alkaline phosphatase and gamma-glutamyl transpeptidase whereas Strain I cells do not. The specific activity of the (Na+ + K+)-ATPase is two-fold greater in Strain II compared with Strain I. The polypeptide composition of the apical membrane differs substantially between the two cell strains as revealed by radio-iodination of external membrane proteins. Monolayer morphology is substantially different between two cell strains. The results are discussed in relation to previous work on MDCK epithelial and the two types of cell monolayer compared with in vivo tubule segments.