A new chloride liquid ion exchanger, Corning 477913, suitable for fabricating ion-selective microelectrodes (ISE) is described. It differs from the standard chloride exchanger. Corning 477315, in that the concentration of organophilic ligand is increased fivefold. The properties of ISE (tip diameter less than 1 micrometer) made from the new material are superior to those of ISE made from the standard material. The yield of acceptable ISE was greater (77 vs. 34%); the ISE resistance was decreased 5.3-fold; the average slope was improved (-52.8 vs. -45.5 mV); and the selectivity over some interfering anions was increased. Measurement of intracellular chloride activity in rabbit ventricle with ISE made from the new exchanger gave results similar to those reported previously with the standard exchanger. This suggests that 477913 is satisfactory for use in biological tissue. The new formulation should be particularly advantageous for measurements requiring ISE with very fine tips and in fabrication of multibarrel ISE.