5-Aminolevulinic acid (ALA)-supported fluorescence endoscopy of the urinary bladder results in a detection rate of bladder cancer superior to that of white light endoscopy, The different accumulation of the metabolite protoporphyrin IX (PPIX) in tumor cells after ALA instillation is poorly understood; however, it is crucial to optimize diagnosis and potential phototherapy, For systematic analysis of cell-type specific PPIX accumulation and metabolism two human bladder carcinoma cell lines (RT4 and J82), a normal urothelial cell line (UROtsa), and a fibroblast cell line (N1) were chosen, and grown in two different growth states to model important tissue components of the urinary bladder, i.e. tumor, normal epithelium and stroma, To quantitate PPIX content, fluorescence intensities measured by flow cytometry were matched with cellular PPIX extraction values, and related to relative ferrochelatase activity, cellular iron content, number of transferrin receptors per cell and porphobilinogen deaminase (PBGD) activity. For in vitro experiments, the initial correlation of relative flow cytometric and spectrometric measurements of PPIX provides a calibration curve for consequent how cytometric PPIX quantification, Lower fluorescence of normal cells could be explained by significant differences of ferrochelatase activity and iron content in comparison to tumor cells. However, the content of iron was not related to transferrin receptor content. PBGD activity seemed to play a minor role for the differential accumulation of PPIX in urothelial cells. In conclusion, the in vitro culture of urothelial cells and fibroblasts indicates that the most important metabolic step for PPIX accumulation in the urinary bladder is the transition from PPIX to heme, Further investigation of PPIX metabolism does support the validation of photodynamic diagnosis, and might also lead the way to a highly specific turner related molecule.