A comparison of time-dependent localization patterns between lower, asymmetrical (AIPCS2a) and higher, symmetrical (AIPCS4) sulfonates of aluminum phthalocyanines in human malignant melanoma LOX transplanted to athymic nude mice from 1 to 120 hr after i.v. administration was made by means of laser scanning fluorescence microscopy. The lipophilic AIPCS2a was distributed mainly in tumor cells, while the hydrophilic AIPCS4 localized only in the vascular stroma of the tumor tissue. Concomitantly, comparative observations on the killing mechanism of photodynamic effects after treatment with a much lower i.v. dose of AIPCS2a and AIPCS4 plus laser light on the human tumor LOX were also made by morphological studies. Light and electron microscopy showed that there was a direct, extensive, photo-damaging action on all organelles and nuclear structure in the tumor cells after PDT with AIPCS2a; whereas the photo-induced injury to the tumor tissue after treatment with AIPCS4 and light was largely the consequence of initial functional vasogenic response and ultimate damage to vascular structure. These findings correlate well with the different localization patterns of the 2 dyes observed in human tumor tissues.