The molecular action mechanism in photodynamic therapy of tumors with porphyrin derivatives (PDT) will be investigated using liposomes and resealed red blood cell membranes (ghosts). The sensitizers to be studied are closely related to the clinical PDT drugs: hematoporphyrin derivative (HpD), the HpD fraction enriched in the tumor-localizing and photosensitizing constituent separated by polyacrylamide gel filtration (HpD-A), and the putative action biologic agent dihematoporphyrin ether (DHE). The liposome studies will include measurements of lipid peroxidation by these porphyrins and their constituents and photosensitized damage to an entrapped enzyme probe. Laser flash photolysis will be used to measure triplet state formation by the porphyrins in liposome membranes and singlet oxygen generation based on the 1.27 micron luminescence. The role of impurity metals on photosensitization by the porphyrins in liposomes will be investigated in connection with the "aging" of porphyrins porphyrins in dilute aqueous media leading to new absorption and fluorescence bands. The studies with ghosts will emphasize the identification of the active oxygen intermediates responsible for lipid peroxidation and protein cross-linking. Prompt damage during irradation and delayed damage during incubation will be delineated. The results obtained by photosensitization with incorporated porphyrins will be augmented by studies in which specific active oxygen agents are generated externally to the ghosts: singlet oxygen, superoxide, hydroxyl radicals and hydrogen peroxide. The mechanisms deducted in these investigations should serve as a basis for the evaluation of photosensitization of cellular systems and indicate the possible mechanisms for photosensitization of membranes in tumor tissue, in which the sensitizers are HpD and its enriched derivatives.