These experiments were performed to test the possibility of a link between involution of the thymus and decreased ability to repair damaged DNA. In a previous investigation this was not found to be the case with thymic lymphocytes, and in the present work the question was addressed to epithelial cells of the thymic stroma isolated by growth in tissue culture. DNA repair in the epithelial cells was measured as unscheduled DNA synthesis (UDS) and detected autoradiographically after UV irradiation of the cultures. In cultures derived from older mice, DNA repair was evident in the majority of the cells and continued after extended culture. When cultures were derived from preinvolution mice DNA repair activity, which was detected after short periods of culture, was lost from most of the cells after several days of growth. These unexpected findings raise the possibility that the ability of the stromal cells to repair DNA damage has enabled them to survive a selective pressure that is involved in thymic involution.