The predominant cell cycle change induced by X-rays and clastogens in peripheral blood mononuclear cells is the accumulation of cells in the G2 phase of the cell cycle. We show that this accumulation consists of cells that are either delayed or arrested within the G2 phase. Since both X-rays and DNA crosslinking chemicals are known to damage DNA, the G2 phase inhibition caused by these agents is thought to be one of the primary manifestations of (unrepaired) DNA damage. This interpretation is supported by two additional findings. (1) Older individuals have elevated baseline levels of mononuclear blood cells that are delayed and/or arrested in the G2 phase of the cell cycle. This coincides with the increased chromosomal breakage rates reported for older individuals. (2) Irrespective of their age, individuals with inherited genetic instability syndromes (such as Fanconi anemia and Bloom syndrome) exhibit elevated G2 phase cell fractions. We show that the method used to detect such induced or spontaneous cell cycle changes, viz. BrdU-Hoechst flow cytometry, is a rapid and highly sensitive technique for the assessment of genetic cell damage.