Cultured cells treated with equal concentrations of thiazine photosensitizers methylene blue (MB) or toluidine blue (TB) showed a distinct photodynamic lethality, with TB being much more effective, when exposed to red light from a LED source. This effect is accounted for because of the differences in the chemical reduction of MB and TB in the intracellular environment. While TB accumulates as blue granular structures, MB does not give such a localization pattern. However, upon exposure of MB-treated cells to oxidant agents, the dye becomes clearly localized in the cytoplasm as blue granules. We propose that massive reduction of MB to its leuco form inside the cell inhibits most of the photodynamic damage, while no such reduction occurs with TB.