Channel access resistance has been measured to estimate the characteristic size of a single ion channel. We compare channel conductance in the presence of nonpenetrating water-soluble polymers with that obtained for polymer-free electrolyte solution. The contribution of the access resistance to the total alamethicin channel resistance is approximately 10% for first three open channel levels. The open alamethicin channel radii inferred for these first three levels from the access resistance are 6.3, 10.3, and 11.4 A. The dependence of channel conductance on polymer molecular weight also allows evaluation of the channel dimensions from polymer exclusion. Despite varying conductance, it was shown that steric radii of the alamethicin channel at different conductance levels remain approximately unchanged. These results support a model of the alamethicin channel as an array of closely packed parallel pores of nearly uniform diameter.