We compared the abilities of structurally related cationic cyclodextrins to inhibit Bacillus anthracis lethal toxin and Staphylococcus aureus ␣-hemolysin. We found that both ␤-and ␥-cyclodextrin derivatives effectively inhibited anthrax toxin action by blocking the transmembrane oligomeric pores formed by the protective antigen (PA) subunit of the toxin, whereas ␣-cyclodextrins were ineffective. In contrast, ␣-hemolysin was selectively blocked only by ␤-cyclodextrin derivatives, demonstrating that both symmetry and size of the inhibitor and the pore are important.Previously, we proposed a novel approach for the discovery of inhibitors of pore-forming toxins that involves the blockage of the pores using molecules with comparable dimensions and the same symmetry as the target pores. It allows for the identification of lead compounds faster and significantly more cheaply in comparison with the existing industry standards. First, this approach was successfully tested on anthrax lethal toxin (LeTx), which plays a key role in anthrax infection. The toxin was disabled by the blockage of the pore formed by protective antigen (PA 63 ), an essential component of anthrax toxin, by rationally designed compounds. Based on the 7-fold symmetry of the PA 63 pore, we synthesized and tested cyclic molecules that had 7-fold symmetry using ␤-cyclodextrin (␤-CD) as a starting molecule (Fig. 1). The discovered inhibitors of anthrax toxin were successfully tested in vitro and in vivo (1-5, 7, 10). The broader applicability of this approach was demonstrated using as targets two other toxins that form transmembrane pores with 7-fold symmetry: ␣-hemolysin (␣-HL) of Staphylococcus aureus (2, 11) and ε-toxin produced by Clostridium perfringens (unpublished data).To investigate how the structural features of the pore blockers affect their activities, we evaluated the ability of structurally related derivatives of ␣-, ␤-, and ␥-cyclodextrins to inhibit the cytotoxic activities of anthrax lethal toxin (LeTx) and staphylococcal ␣-HL as well as to block the ion current through the channels formed by PA 63 and ␣-HL in planar lipid membranes.