Most sensitizers used for the photodynamic therapy (PDT) of tumors photobleach on illumination. Thus, it is of interest to examine the photobleaching behavior of new sensitizers proposed for use in PDT. This report surveys the quantum yields and kinetics of the photobleaching of mono-L-aspartyl chlorin e6(NPe6), a hydrophilic chlorin that has many of the photoproperties desirable in a sensitizer for clinical PDT. It is a very effective sensitizer for the PDT of several types of model tumors in animals and is now in Phase I clinical trials. The quantum yield of NPe6 photobleaching in pH 7.4 phosphate buffer in air was 8.2 x 10(-4); this is greater than the yields for typical porphyrin photosensitizers. For example, the yields for hematoporphyrin and uroporphyrin are 4.7 x 10(-5) and 2.8 x 10(-5), respectively. The yield decreased significantly in organic solvents of low dielectric constant. The Sn derivative of NPe6 was more light stable than NPe6 (yield = 5.7 x 10(-6), while the Zn derivative was more sensitive (yield = 1.9 x 10(-2). Oxygen appeared to be necessary for the photobleaching of NPe6; however, bleaching was not inhibited by 100 mM azide, an efficient quencher of singlet oxygen. The photooxidizable substrates cysteine, dithiothreitol and furfuryl alcohol increased the quantum yield of photobleaching two- to four-fold, while the electron acceptor, metronidazole, increased it almost six-fold. Photobleaching yields for several other chlorins were also measured.