We report that the photosensitizer verteporfin kills lymphoma cells by an apoptotic process involving a dissipation of the mitochondrial inner transmembrane potential (ΔΨm). Light-activated verteporfin-induced apoptosis was abolished by transfection with Bcl-2,a procedure reported to inhibit the mitochondrial permeability transition pore complex (PTPC). Verteporfin triggered the ΔΨm loss in isolated mitochondria in vitro, and this effect was suppressed by bongrekic acid and cyclosporin A. Verteporfin plus light also permeabilized proteoliposomes containing the semipurified PTPC or the purified PTPC component adenine nucleotide translocator (ANT), yet had no effect on protein-free control liposomes. Verteporfin phototoxicity on ANT proteoliposomes was mediated by reactive oxygen species and was prevented by recombinant Bcl-2 or the adenine nucleotides ATP and ADP. In conclusion, verteporfin belongs to a class of clinically used chemotherapeutic agents acting on PTPC and ANT.