Nuclear pore complexes (NPCs) restrict uncontrolled nucleocytoplasmic fluxes of inert macromolecules but permit facilitated translocation of nuclear transport receptors and their cargo complexes. We probed the passive barrier of NPCs and observed sieve-like properties with a dominating mesh or channel radius of 2.6 nm, which is narrower than proposed earlier. A small fraction of diffusion channels has a wider opening, explaining the very slow passage of larger molecules. The observed dominant passive diameter approximates the distance of adjacent hydrophobic clusters of FG repeats, supporting the model that the barrier is made of FG repeat domains cross-linked with a spacing of an FG repeat unit length. Wheat germ agglutinin and the dominant-negative importin beta(45-462) fragment were previously regarded as selective inhibitors of facilitated NPC passage. We now observed that they do not distinguish between the passive and the facilitated mode. Instead, their inhibitory effect correlates with the size of the NPC-passing molecule. They have little effect on small species, inhibit the passage of green fluorescent protein-sized objects >10-fold and virtually block the translocation of larger ones. This suggests that passive and facilitated NPC passage proceed through one and the same permeability barrier.