The water-soluble porphyrin-platinum complex diammine[7,12-bis[1-(polyethyleneglycol-750-monomethylether-1-yl)ethyl]-3,8,13,17-tetramethylporphyrin-2,18-dipropionato]platinum(II) (PEG-HPPt) was studied with respect to cellular accumulation, subcellular localization, behavior in 3D-cell aggregates and degree of DNA platination on the low-differentiated J82 cells, a model of invasive bladder cancer, and UROtsa, a normal urothelial cell line. Accumulation studies with 2D and spheroid cell cultures revealed that the concentration of PEG-HPPt was 1.7-times higher in J82 cancer cells than in UROtsa cells. Despite its high molecular weight, penetration of PEG-HPPt was not restricted to the peripheral cells of the spheroids. Fluorescence microscopic analysis showed that PEG-HPPt was localized in essential cellular targets of photodynamic therapy. DNA platination in J82 and UROtsa cells was higher by PEG-HPPt than by cisplatin, whereas there was no significant difference between the two cell lines.