Abstract—Photosensitization mediated by Photofrin II (PFII) was found to be mutagenic at the heterozygous thymidine kinase (tk) locus in mouse L5178Y lymphoma strains LY‐S1 and LY‐R16 but not in strain LY‐R83 which is hemizygous at thetklocus. After treatments yielding 37% survival, the mutagenicity of photosensitization with PFII in strain LY‐S1 was similar to that of other mutagenic agents including x‐radiation, ethyl methanesulfonate, and photosensitization with chloroaluminum phthalocyanine (AIPcCI). Although both strain LY‐S1 and strain LY‐R16 were mutagenized by photosensitization with PFII, only strain LY‐S1 was mutagenized by photosensitization with AIPcCI. The non‐mutability of strain LY‐R83 following photodynamic treatment with either sensitizer may be because of the poor recovery of mutants with intergenic mutations in this TK+/0hemizygous strain, whereas the non‐mutability of strain LY‐R16 subjected to photodynamic treatment with AIPcCI may be because LY‐R16 cells sustaining mutagenic damage do not survive for reasons other than th