Nine dyes, all potential sensitizers for photodynamic cancer therapy (PDT), were injected in mice with C3H mammary carcinomas. Twenty-four hours later the animals were sacrificed and the dye concentrations in tumors and 9 other tissues were measured by means of spectrofluorimetry. The 9 dyes were: Photofrin II (PII), hematoporphyrin (HP)-di-hexyl-ether, PSD-007 (a sensitizer used in clinical trials in China), tetraphenyl porphine tetrasulfonate (TPPS4), tetra(3-hydroxy phenyl)porphyrin (3THPP), aluminium phthalocyanine tetrasulfonate (AlPCTS), aluminium phthalocyanine (AlPC), chlorin e6 (Chl e6) and merocyanine 540 (MC 540). The porphyrin precursor delta-aminolevulinic acid was also tested and found to induce porphyrin fluorescence in tumors and some other tissues. The best tumorlocalizer of those tested was 3THPP. This drug also showed a favorable tissue distribution. The following dyes showed lower skin/tumor concentration ratios than PII (the most widely used dye for PDT): Chl e6, PSD-007, HP-di-hexyl-ether and 3THPP. Low brain/tumor ratios were found for: PSD-007, HP-di-hexyl-ether, 3THPP, TPPS4 and AlPCTS.