The intracellular localization and the photosensitizing effects of AIPCS₄ in cells in vitro were studied by means of fluorescence microscopy, absorption - and fluorescence spectroscopy and cell survival measurements. This hydrophilic dye is partly aggregated in the cells. In cells kept in the dark the dye is to a large extent localized in lysosomes. Upon light exposure, the lysosomes rypture, whereupon the local concentration of dye decreases. This leads to a large increase in the fluorescence, quantum yield. Furthermore, the light exposure leads to a striking relocalization of the dye in the cells; the nuclei, which in unexposed cells show minimal fluorescence, become brightly fluorescing. The light induced fluorescence increase is dependent on the dye concentration as well as on the fluence. By means of survival experiments it was attempted to evaluate the significance of these observations for the photoinactivation of cells.