The Ca(+)-activated K+ channel of the human red cell membrane was characterized with respect to rectification and selectivity using the patch-clamp technique. In inside-out patches exposed to symmetric solutions of K+, Rb+, and NH+4, respectively, inward rectifying i-V curves were obtained. The zero current conductances were: K+ (23.5 pS +/- 3.2) greater than NH+4 (14.2 pS +/- 1.2) greater than Rb+ (11.4 pS +/- 1.8). With low extracellular K+ concentrations (substitution with Na+) the current fluctuations reversed close to the Nernst potential for the K ion and the rectification as well as the i-V slopes decreased. With mixed intracellular solutions of K+ and Na+ enhanced rectification were observed due to a Na+ block of outward currents. From bi-ionic reversal potentials the following permeability sequence (PK/PX was calculated: K+ (1.0) greater than Rb+ (1.4 +/- 0.1) greater than NH+4 (8.5 +/- 1.3) greater than Li+ (greater than 50); Na+ (greater than 110); Cs+ (much greater than 5). Li+, Na+, and Cs+ were not found to carry any current, and only minimum values of the permeability ratios were estimated. Tl+ was permeant, but the permeability and conductance were difficult to quantify, since with this ion the single channel activity was extremely low and the channels seemed to inactivate. The inward rectification in symmetric solutions indicate an asymmetric open channel structure, and the different selectivity sequences based on conductances and permeabilities reflect inter-ionic interactions in the permeation process.