Abstract— A series of photosensitizers known to cause photodermatitis were tested with respect to photosensitizing properties. Irradiation of these sensitizers together with unsaturated fatty acids or amino acids showed that few of them can be transformed by most of the photosensitizers. SH groups can. however, be destroyed by most of the photosensitizers known in dermatological practice. At low photosensitizer‐to‐SH group ratios, GSSG is formed; at higher ratios, other transformations of GSH predominates. The reactivity of GSH with ninhydrin is decreased after irradiation with a photo‐sensitizer. Glutathione seems to be a biomolecule that is suitably sensitive to photosensitized modification and thus suitable for evaluation of any photosensitizing activity that new drugs may have as an unwanted side effect. Copyright © 1979, Wiley Blackwell. All rights reserved