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 P-II (Photofrin II), 3-THPP tetra(3-hydroxyphenyl)porphin, TPPS1 (tetraphenylporphine monosulfonate), TPPS2a (tetraphenylporphine disulfonates with the sulfonate groups on adjacent rings), A1PCS1 (aluminium phthalocyanine monosulfonate) and A1PCS2 (aluminium phthalocyanine disulfonates) localized mainly in tumor cells. The fluorescence intensity of these dyes increased from 4 h to 48 h post-injection and the fluorescence was still observable 120 h post-injection. The more hydrophilic dyes such as TPPS2o (tetraphenylporphine disulfonates with the sulfonates groups on opposite rings), TPPS3 (tetraphenylporphine trisulfoantes), TPPS4 (tetraphenylporphine tetrasulfonates), A1PCS3 (aluminium phthalocyanine trisulfonates) and A1PCS4 (aluminium phthalocyanine tetrasulfonates) localized mainly extracellularly in the tumorous stroma. The fluorescence intensity of these dyes decreased from 4 h to 48 h post-injection. 120 h post-injection no significant fluorescence of these dyes could be seen in the tumors. The data are discussed in relation to what is known about the in vivo photosensitizing efficiency of some of the dyes.