[en] The dimensions of DNA fragments present in polydeoxynucleotide (PDN), obtained after degradation of irradiated rat thymus chromatin or after digestion of chromatin products in vitro using Ca²⁺, Mg²⁺ -dependent- and micrococcal endonucleases have been investigated, by comparison of the buoyant densities of the deproteinized preparation of PDN and the products of chromatin treatment with endogenous nucleases. The measurements of the molar fraction of GC-pairs showed that the content of guanine and cytosine in PDN was 42.2% and 41.3%, respectively, 1 and 4 hours after X-irradiation (400R). It is suggested that PDN is represented by fragments which are produced by a regular mechanism of post-irradiation degradation of chromatin and correspond to monomeric nucleosomes and their oligomers, and that chromatin breakdown is caused by nucleases which become activated in cells exposed to radiation. (U.K.)