Subcellular localization of meta-tetra (hydroxyphenyl) chlorin (mTHPC) in HT29 human colon adenocarcinoma cells has been studied by means of fluorescence microscopy. The observed diffuse intracellular distribution of mTHPC fluorescence outside the nucleus indicates general staining of cellular organelles. No changes in dye fluorescence pattern are evident during and immediately after cell illumination. Alternatively, the changes in mTHPC fluorescence pattern are observed upon subsequent cell incubation, and are characterized by the appearance of distinct bright fluorescence zones. Reaching a maximum 1 h after illumination, modifications of the fluorescence pattern then gradually disappear in parallel with the formation of plasma membrane blebs, suggesting that cell necrotic lysis is taking place. Photosensitized damage to mitochondria and the Golgi apparatus has been studied using fluorescent probes 1 h after irradiation, the stage of extensive cytoplasm vacuolization, and reveal alterations of these organelles. Changes in the mTHPC fluorescence pattern and mTHPC photocytotoxicity, as measured by the MTT test 24 h after illumination, are inhibited by sodium azide, a singlet oxygen quencher.