Mastoparan (MP), a potent facilitator of mitochondrial permeability transition (PT), could be used as an antitumor agent, if it were encapsulated in a tumor selective delivery system. We recently developed transferrin-modified liposomes (Tf-L) with a pH-sensitive fusogenic peptide (GALA), which delivers an encapsulated fluorescent marker into cytosol efficiently. In this study, we encapsulated MP into Tf-L with GALA for the selective delivery to mitochondria of tumor cells. The MP showed potent PT activity at concentrations above 25 microM in a homogenate of K 562 cells as well as in isolated mitochondria in the presence of phosphate. Tf-L equipped with cholesteryl GALA can release encapsulated sulforhodamine B, while Tf-L failed, as evidenced by confocal laser scanning microscopy. The MP, which was delivered with Tf-L with GALA, released cytochrome c (cyt c) from mitochondria to the cytosol, while free MP released cyt c not only to the cytosol but also extracellulary. These results demonstrate the utility of MP in Tf-L with GALA for cancer therapy.