Multiphoton imaging of the sub-cellular distribution of photosensitizers can provide important clues to their mechanism of action in tumors. We have used this tool to study distribution and pharmacology of photosensitizers in murine hepatoma tumor cells dosed with various photosensitizers. Upon photoactivation, Rose Bengal yields nearly immediate photolytic release of lysomal enzymes, resulting in catastrophic cell destruction within 5-30 minutes. Such marked response is quite different than that observed with other photosensitizer agents, and is consistent with in vivo studies illustrating that Rose Bengal is capable of causing extremely rapid destruction of treated tumors.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.