The dot-blot method described here can be used to measure repair of pyrimidine-pyrimidone 6-4 photoproducts ([6-4]PPs) and cyclobutane pyrimidine dimers (CPDs) in total genomic DNA from any organism. The DNA does not have to be especially intact nor is it necessary to have any sequence information. Although the protocol given below is for measuring repair in budding yeast, the method has been successfully used to measure repair in fission yeast, human cells, archaebacteria, Streptomyces, Aspergillus, and plants (1–3; and McCready, unpublished). The assay is sufficiently sensitive to measure damage induced by 10 J/m² of UV-C with ease and could be used for lower doses.