TcdB (2366 amino acid residues) is an intracellular bacterial toxin that binds to cells and enters the cytosol where it glucosylates small GTPases. In the current study， we examined a putative cell entry region of TcdB (amino acid residues 1753-1851) for short sequences that function as cell-penetrating peptides (CPPs). To screen for TcdB-derived CPPs， a panel of synthetic peptides was tested for the ability to enhance transferrin (Tf) association with cells. Four candidate CPPs were discovered， and further study on one peptide (PepB2) pinpointed an asparagine residue necessary for CPP activity. PepB2 mediated the cell entry of a wide variety of molecules including dextran， streptavidin， microspheres， and lentivirus particles. Of note， this uptake was dramatically reduced in the presence of the Na/H exchange blocker and micropinocytosis inhibitor amiloride， suggesting that PepB2 invokes macropinocytosis. Moreover， we found that PepB2 had more efficient cell-penetrating activity than several other well-known CPPs (TAT， penetratin， Pep-1， and TP10). Finally， Tf assay-based screening of peptides derived from two other large clostridial toxins， TcdA and TcsL， uncovered two new TcdA-derived CPPs. In conclusion， we have identified six CPPs from large clostridial toxins and have demonstrated the ability of PepB2 to promote cell association and entry of several molecules through a putative fluid-phase macropinocytotic mechanism.