Xeroderma pigmentosum (XP) is a hereditary disease characterized by a defect in the excision-repair mode of ultraviolet light damage and a high incidence of skin tumors. Cultured fibroblasts from normal and XP cells at low population doubling times were compared by induction of mild spreading of their nuclear constituents in a highly alkaline solution containing detergent and formaldehyde. In each XP culture a certain fraction (10-80%) of the nuclei were abnormal (50-80% in cell lines from children with XP-C disorders and 10-35% from embryonic and adult XP cells). Although their chromatin threads appeared normal in structure, they were separated by intervals up to 5 times the normal spacing. In all XP cells having this abnormal spacing in the chromatin, fibrils of nucleolar origin were approximately doubled in thickness, denser and less tufted, and nucleolar granules were few and dispersed. We suggest that this study reveals an abnormal weakness of the chromatin in some XP cells which results in the breakage of some DNA fibers in our preparative alkaline conditions. This weakness may be related to single-stranded breaks induced by metabolism of a high level of active oxygen species. These nuclear changes in XP cells are similar to those which have been associated with normal or pathologic senescence.