The photodynamic effect of meso-substituted cationic porphyrins, 5-[4-(trimethylammonium)phenyl]-10,15,20-tris(2,4,6-trimethoxyphenyl)porphyrin iodide 1, 5,10-di(4-methylphenyl)-15,20-di(4-trimethylammoniumphenyl)porphyrin iodide 2 and 5-(4-trifluorophenyl)-10,15,20-tris(4-trimethylammoniumphenyl)porphyrin iodide 3, have been investigated in both homogeneous medium bearing photooxidizable substrates and in vitro on a typical gram-negative bacterium Escherichia coli. Absorption and fluorescence spectroscopic studies were compared in N,N-dimethylformamide. Fluorescence quantum yields (varphiF) of 0.10, 0.06 and 0.08 were calculated for porphyrins 1, 2 and 3, respectively. The singlet molecular oxygen, O2(1Deltag), production was evaluated using 9,10-dimethylanthracene yielding values of 0.66, 0.36 and 0.42 for porphyrins 1, 2 and 3, respectively. Guanosine 5'-monophosphate was used as biological substrate model. Similar decomposition of guanosine 5'-monophosphate was obtained using these cationic porphyrins as sensitizer. In biological medium, photosensitized inactivation of E. coli was analyzed using cells without and with one washing step. E. coli cultures were treated with sensitizer at 37 degrees C for 30 min in dark. In both procedures, a higher photoinactivation of cells (>99.999%) was found for cells treated with 10 microM of tricationic porphyrin 3 and irradiated for 5 min with visible light. Porphyrins 1 and 2 only show an important photodamage when the cells are irradiated without washing step. These results indicated that the tetracationic porphyrin 3 could be a promising sensitizer with potential applications in the photoinactivation of bacterial cells by photodynamic therapy.