This proposal is a request for NIH support for a Workshop on use of porphyrins for tumor localization and photosensitization. During the past several years, the porphyrin product HPD has been found capable of concentrating in tumor loci. Neoplastic lesions can be identified by the resulting fluorescence, and selectively destroyed, via porphyrin-catalyzed energy transfer, upon irradiation with red light. Periodic conferences are useful for exchange of information among workers in the field, and for providing updates of pre-clinical and clinical results for the benefit of individuals who are beginning research on photosensitization phenomena. The past three such conferences in the US were held in September 1981 (with NIH support), in April 1983 and in July 1984 (with NIH support). These were all successful meetings, providing a useful format for the goals outlind above. All resulted in publication of books containing contributions from the participants; these form the basis for the reference material in this field. There have also been foreign conferences in Milan (1983) and in Sardinia (1985). During the next such meeting, it is proposed that workers from clinical and preclinical research groups be invited, along with experts in porphyrin chemistry and lipoprotein biology. The latter topic derives from some recent information which suggests that porphyrin-lipoprotein interactions provide the basis for the tumor-localization phenomena. Numbers of LDL receptors in different tissues are correlated with porphyrin accumulation. This continued exchange of information is essential in order that progress in this new anti-tumor modality be maintained.