Hepatic microsomes prepared from rats pretreated with hematoporphyrin derivative (HPD) undergo rapid enhancement of lipid peroxidation in the presence of solar radiation (~400 nm). Quenchers of singlet oxygen, including 2,5-dimethylfuran, histidine, and 0-carotene, and inhibitors of the hydroxyl radical, including benzoate, mannitol, and ethanol, largely protected against the enhancement of lip...