Electron paramagnetic resonance (EPR) experiments were performed on Victoria Blue BO, a cationic dye whose photocytotoxicity has been studied against the human leukaemic cells lines K-562 and TF-1. EPR experiments with 2,2,6,6-tetramethyl-4-piperidone and spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide showed that, on illumination in aerated aqueous solution ordl -α-dipalmitoylphosphatidylcholine liposomes, photoexcited Victoria Blue BO is unable to generate 1 O 2 , whereas O 2 - and OH are trapped by 5,5-dimethyl-1-pyrroline-N-oxide in the presence or absence of electron donors. The O 2 - formed probably leads to the OH radical, with an efficiency which is increased by electron donors such as Fe 2 + .