The rate of photohaemolysis of human red blood cells sensitized by chloroaluminium phthalocyanine sulphonate is increased by ascorbate, with or without added FeCl3. Stimulation of haemolysis by ascorbate without addition of metal salt, and in the presence of a strong chelator such as desferrioxamine, is an unexpected phenomenon. Lysis rate and ascorbate concentration were directly related, suggesting that ascorbate acts as a reactant and not as a catalyst. The process also requires oxygen; azide and D2O tests indicate some participation of singlet oxygen, although to a lesser extent than in the photosensitized haemolysis in the absence of ascorbate. Kinetic considerations suggest a reaction path initiated by excited sensitizer and ascorbate, parallel to the singlet oxygen-mediated process. Because of the ubiquitous presence of ascorbate in human tissues in concentrations comparable to those of dissolved oxygen, it is quite possible that in photodynamic therapy a fraction of the photodynamic damage proceeds via a Type I, ascorbate-assisted, mechanism.