The DNA and histone content of HeLa S-3 cell cultures was measured by direct mass assays 21 hours after 1000 rad of X-irradiation, when the cells were arrested in G2 phase. The nuclear DNA content of such cultures was found to be deficient (73% of control values). In contrast, the synthesis of nuclear histones persisted, and the total histone content was close to 100% of control values. When synchronously-growing cultures were irradiated in mid-S phase and examined 3-5 hours later in G2 phase, both DNA and histone content were equal to control values.