Abstract— The formation of thymine dimers in the DNA of L‐strain mammalian cells after irradiation with ultraviolet light has been demonstrated. The amount of dimer formed rises with the dose of u.v. light. In the course of post‐irradiation incubation the thymine dimers remain in the TCA insoluble fraction and diminish as did the other thymidine‐H3 derivatives with increasing incubation time. The dimer is not found in the soluble fraction. Thus, dimer excision (i.e. its liberation into the soluble fraction) as an expression of repair of radiation damage analogous to dark repair in E. coli was not found in these experiments.

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