Ultraviolet irradiation of DNA results in various pyrimidine modifications. The major products are cyclobutane dimers and 6-(1,2-dihydro-2-oxo-4-pyrimidinyl)-5-methyl-2,4(1H,3H)pyrimidinediones, both formed at sites of adjacent pyrimidines on the same strand. Other photoproducts result from addition of water at the 5,6-double bonds of DNA pyrimidines. These hydrates are formed at cytosines and uracils and, to a far lesser extent, at thymines. The repair-ability of these pyrimidines hydrates suggests them to be stable in DNA. All such 6-hydroxy-5,6-dihydropyrimidines are excised from DNA as free bases by enzymes found in a wide variety of species, including human cells. These glycosylases are best assayed by direct measurements of the released bases from appropriately radiolabeled alternating DNA copolymers. They are active against both right-handed and left-handed DNA helices. Levels of these enzymes are increased in dividing cells, but are unaltered in cells as they approach or attain senescence. Since these enzymes have a catholicity of substrates, releasing a plethora of ring-saturated or ring-opened pyrimidines from DNA, they are of importance in the repair of oxidative damages to DNA as well as in excision of pyrimidine hydrates.