This program is directed towards understanding the photosensitized reactions of biological target molecules, with current emphasis on furocoumarin compounds employed for PUVA therapy of human skin diseases. The possible involvement of singlet oxygen generated by furocoumarins in membrane damage will be investigated using liposomes and red blood cells. The current studies on dark complexing of furocoumarins to DNA and the effects on the primary photosensitization pathways will be investigated with liposome-DNA systems. The mathematical modeling of furocoumarin reaction kinetics will be continued, emphasizing the pathways for free and complexed furocoumarin molecules.