A new Cl- selective microelectrode based on the ionophore 5,10,15,20-tetraphenyl-21H,23H-porphin manganese(III) chloride is presented which discriminates better against HCO3- and several organic anions than electrodes containing the Corning 477913 ion-exchanger. Using a redesigned construction procedure, fine-tip double-barrelled microelectrodes were produced which had slopes of -52.4 +/- 0.6 mV (SE, n = 24), resistances of about 7.10(11) omega and a selectivity coefficient log KpotCLHCO3 of -1.40 +/- 0.03. Some electrodes showed a small unexplained sensitivity to pH greater than 7.6. When used to puncture cells of isolated S3 segments of rabbit renal proximal tubule during perfusion with HCO3- Ringer solution, the electrodes gave a membrane potential of -69.8 +/- 1.5 mV and an intracellular Cl- activity, [Cl-]i, of 35.3 +/- 2.6 mmol/l. Upon switching bath and lumen perfusions to Cl- -free solutions the "residual" [Cl-]i dropped to 1.20 +/- 0.03 mmol/l, while in similar measurements with ion-exchanger electrodes the "residual" [Cl-]i dropped only to 10.9 +/- 0.5 mmol/l. These observations demonstrate the superiority of the new electrode and prove that previously determined high [Cl-]i values in Cl- -free ambient solutions reflect interference problems rather than non-exchangeable intracellular chloride.