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Meta-tetra(hydroxyphenyl)chlorin (mTHPC) is in clinical trials for the photodynamic therapy (PDT) of localized-stage cancer. The PDT susceptibility of cells expressing multidrug resistance (MDR) phenotype is an attractive possibility to overcome the resistance to cytotoxic drugs observed during cancer chemotherapy. The accumulation, photocytotoxicity and intracellular localization of mTHPC were examined using the doxorubicin selected MCF-7/DXR human breast cancer cells, expressing P-glycoprotein (P-gp), and the wild-type parental cell line, MCF-7. No significant difference in mTHPC accumulation was observed between the two cell lines up to 3 h contact. The photodynamic activity of mTHPC, measured 24 h after irradiation with red laser light (λ=650 nm), was significantly greater in MCF-7/DXR as compared to MCF-7 cells. A light dose of 2.5 J cm⁻² inducing 50% of cytotoxicity in MCF-7, resulted in 85 …