By means of laser scanning fluorescence microscopy the intratumoral localization patterns of several photosensitizers in LOX tumors in nude mice were studied. Lipophilic dyes such as TPPS1 (tetraphenylporphine monosulfonate), TPPS2a (tetraphenylporphine disulfonates with the sulfonate groups on adjacent rings), AlPCS1 (aluminium phthalocyanine monosulfonate) and AlPCS2 (aluminium phthalocyanine disulfonates) localized mainly in tumor cells. The fluorescence intensity of these dyes increased from 4 h to 48 h postinjection and the fluorescence was still observable 120 h postinjection. The more hydrophilic dyes such as TPPS3 (tetraphenylporphine trisulfonates), TPPS4 (tetraphenylporphine tetrasulfonates), and AlPCS4 (aluminium phthalocyanine tetrasulfonates) localized mainly extracellularly in the tumorous stroma. The fluorescence intensity of these dyes decreased from 4 h to 48 h postinjection. 120 h postinjection no significant fluorescence of these dyes could be seen in the tumors. P-II (Photofrin II), 3-THPP [tetra(3-hydroxyphenyl)porphine], TPPS2o (tetraphenylporphine disulfonates with the sulfonate groups on opposite rings) and AlPCS3 (aluminum phthalocyanine trisulfonates) had a combined localization pattern, i.e. a strongly cytoplasmic membrane-localizing pattern and a weakly intracellular distribution pattern, although some fluorescence could be seen in the tumorous stroma. The data are discussed in relation to what is known about the in vivo photosensitizing efficiency of some of the dyes.