The efficiency of radiation-induced production of 5'',6''-dihydroxydihydrothymine (t.gamma.)-type damage was determined in nascent and mature chromatin DNA [from human cervical carcinoma HeLa S3 cells] for the dose range of 50-150 krad. These large doses affected neither the total fraction of nuclear DNA in chromatin subunits nor the nucleosome subunit repeat length. The DNA in nascent chromatin, however, was 3.3 times more sensitive than mature chromatin DNA to .gamma.-ray (137Cs)-induced t.gamma.-type damage, while thymine damage of this type was uniformly distributed in the nucleosomal DNA of mature chromatin (i.e., in the nucleosome core and spacer DNA). The half-time for the transition of nascent DNA sensitivity to mature chromatin DNA sensitivity levels was the same as the half-time at 37.degree. C for the maturation of nascent into mature chromatin structure. The rate at which nascent chromatin matured was unaffected by radiation doses as large as 150 krad. The most logical explanation for the greater sensitivity of nascent DNA to radiation is the decreased concentration of histone chromosomal proteins in nascent chromatin.