Liquid ion-exchanger microelectrodes based on Corning code 477317 K+ exchanger are known to be much more sensitive to quaternary ammonium ions than to K+. In the presence of such cations, the capability of measuring K+ activities with Corning microelectrodes may be seriously impaired. We have developed a neutral carrier K+-selective microelectrode based on the crown ether dibenzo-18-crown-6. The crown ether cocktail contained (wt/wt) 2.3% dibenzo-18-crown-6, 0.8% Na-tetraphenylborate, 30.1% 2-nitrophenylocylether, and 66.8% O-nitrotoluene. Double-barreled crown ether and Corning microelectrodes were calibrated in KCl solutions with or without choline, acetylcholine, tetramethylammonium, imidazole, Na+, tris(hydroxymethyl)aminomethane (Tris), and N-methyl-D-glucamine. Both kinds of microelectrodes showed similar K+ over Na+, Tris, and N-methyl-D-glucamine selectivities. However, crown ether microelectrodes had immensely greater selectivities of K+ over quaternary ammonium ions and imidazole than Corning microelectrodes. Selectivity factors, defined as log K(ij)K, of crown ether microelectrodes with respect to K+ for tetramethylammonium, choline, acetylcholine, and imidazole were -1.92 +/- 0.13, -2.97 +/- 0.03, -1.75 +/- 0.15, and -1.30 +/- 0.20, respectively. Intracellular K+ activities measured in the same Necturus gallbladders with both kinds of microelectrodes did not differ significantly.