Abstract: A simple and fast process is described which serves to directly determine apurinic and apyrimidinic sites (AP sites) in DNA using [.sup.14 C]methoxyamine which, after loss of a purine or pyrimidine base, respectively, reacts with the resultant aldehyde groups of the deoxyribose groups.The incorporation of the [.sup.14 C]methoxyamine in DNA is proportional to the number of AP sites. Since methoxyamine does not cause the DNA to degrade, the unreacted AP sites can be measured in order to determine the radioactivity of the acid-insoluble fraction.The process lends itself to the analysis of DNA-damages such as those which are physically or chemically produced. Moreover it is suitable to demonstrate and determine the activity of DNA glycosylases, in particular of uracil-DNA glycosylases.