On the basis of intracellular, accumulation of c-SNAFL-2, we have identified three cell subtypes in Madin-Darby canine kidney (MDCK) monolayers. Highly fluorescent cells (HFC) have a high intracellular pH (pHi, whereas cells with medium fluorescence (MFC) have low pHi when perfused with buffer containing 125 mM Cl-. HFC express a Cl-/HCO3- exchanger on the apical but not the basolateral membrane. MFC express a Cl-/HCO3- exchanger on the basolateral but not the apical membrane. We have termed these cells beta- and alpha-MDCK cells, respectively. Cells with low fluorescence (LFC) probably extrude c-SNAFL-2 through a monocarboxylate transporter, because p-4-(chloromercuri)phenylsulfonic acid (PCMBS), an inhibitor of this transporter, leads to homogeneous fluorescence.