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Abstract Penetration potency of δ-aminolevulinic acid (ALA) was studied by examining fluorescence of endogenous protoporphyrin IX in different histological types of basal cell carcinoma. Ten basal cell carcinomas were coated with an ointment containing 10% ALA prior to excision; five served as controls. Tumors were excised either 4 h or 12 h after application of ALA using a modified Mohs’micrographic surgical technique. Horizontal sections were cut from deep dermis to tumor surface and examined under a fluorescence microscope. After 4 h of application, only skin appendages demonstrated fluorescence typical of protoporphyrin IX. After 12 h, fluorescence was detectable in tumor cells in deep dermis. The five controls revealed no fluorescence at any site. These results may confirm the high penetration potential of topically applied ALA and its usefulness in photodynamic therapy. For tumors penetrating to deep dermis, an application time of more than 4 h seems necessary, at least when hydrophilic solvents for ALA are used.