Tumor uptake and fluorescence characteristics of some phthalocyanines were studied in vivo by means of laser irradiation and fluroscence spectroscopy. In contrast to what is found in vitro, the sensitizers show a decrease in fluorescence during light exposure in vivo. Surprisingly the fluorescence was found to recover almost to the initial value when the laser light was turned off. Irradiation of an aqueous solution of the sensitizer in the presence of different amounts of serum indicated that this phenomena was related to binding of the sensitizers to proteins.