Chloroaluminum phthalocyanine tetrasulfonate (Al-PcTS) is a promising photosensitizer for the photodynamic therapy (PDT) of cancer. In this study, we investigated the in vivo and in vitro photodestruction of hepatic microsomal membranes by AlPcTS and studied the role of reactive oxygen species in this process. Irradiation of hepatic microsomes prepared from AlPcTS-pretreated SENCAR mice to ≈675 nm light resulted in rapid destruction of cytochrome P450 and associated monooxygenase activities, and enhancement of lipid peroxidation in a light-dose-dependent manner. The specificity of AlPcTS and light dependency on photodestruction of microsomal membranes was confirmed by Western blot analysis. Similar results were obtained when AlPcTS was added in vitro to a suspension of hepatic microsomes prepared from control animals followed by irradiation to ≈675 nm light. Among the quenchers of singlet oxygen, superoxide anion, hydrogen peroxide, and hydroxyl radical, only the quenchers of singlet oxygen such as sodium azide, histidine, and 2,5-dimethyl furan afforded substantial protection in a dose-dependent manner against AlPcTS-mediated photodestruction of cytochrome P450 and associated monooxygenase activities, and photoenhancement of lipid peroxidation under both in vivo and in vitro conditions. These results suggest that lipid-rich microsomal membranes may be the potential targets of cell injury by AlPcTS-based PDT and that this process is mediated by singlet oxygen. © 1992.