Cytotoxicity of sulfonated Chlor-.Aluminium Phthalocyanine (C1A1SPc) was investigated for 2 bladder tumor cell lines and for normal bladder wall cells as control. The particular interest has been focussed on the correlation of the sensitizer's uptake with the incubation time and on the subcellular morphologic changes following PDT. Cells were cultivated in monolayer cultures using DMEM + 15 % FCS. For the assessment of the cellular uptake, the cells were incubated with 40 micro-g CIAISPc for 2, 4, 6 and 24 hours respectively. The sensitizer was then extracted from the cells using 3ml 0.1 m NaOH for 1 hour at 37 °C. Concentration of the sensitizer was then calculated measuring absorption of the extracted substance at 670 nm with a Zeiss photometer. To evaluate the subcellular changes, an incubation time of 4 hours was chosen. Following PDT (100 mW/cm2 for 2 minutes) the cells were assessed under the electron microscope in transmission technique. To identify and assess the mitochondria in particular, cells were stained with DAB. As expected, the uptake of CIA1SPc in vitro depends on the incubation time. Rapid increase in concentration can be demonstrated from 2 to 4 hours incubation time, whereas following 6 and 24 hours the increase is less marked. Nevertheless, a short incubation of 2 or 4 hours, which would be realistic for a topic application of the sensitizer under clinical conditions (bladder tumors!), already yields a significant and probably sufficient intracellular concentration for subsequent PDT. Following PDT, most of the investigated cells show significant changes of the mitochondria which is confirmed by DAB staining. The number of intact mitochondria in treated cells is significantly smaller than in non-treated cells.