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Toxins could be effective anticancer drugs, if their selective delivery into cancer cells could be achieved. We have shown that the energy of membrane-associated folding of water-soluble membrane peptides of the pHLIP (pH low insertion peptide) family could be used to move cell-impermeable cargo across the lipid bilayer into the cytoplasm of cancer cells. Here we present the results of a study of pHLIP-mediated cellular delivery of a polar cell-impermeable toxin, α-amanitin, an inhibitor of RNA polymerase II. We show that pHLIP can deliver α-amanitin into cells in a pH-dependent fashion and induce cell death within 48 h. Translocation capability could be tuned by conjugating amanitin to the C-terminus of pHLIP via linkers of different hydrophobicities that could be cleaved in the cytoplasm. pHLIP-SPDP-amanitin, which exhibits 4–5 times higher antiproliferative ability at pH 6 than at pH 7.4, was selected as the best construct. The major mechanism of amanitin delivery is direct translocation (flip) across a membrane by pHLIP and cleavage of the S–S bond in the cytoplasm. The antiproliferative effect was monitored on four different human cancer cell lines. pHLIP-mediated cytoplasmic delivery of amanitin could create great opportunities to use the toxin as a potent pH-selective anticancer agent, which predominantly targets highly proliferative cancer cells at low extracellular pH values.