DNA single-strand break and rejoin were assayed in normal fibroblast ataxia telangiectasia (AT) and AT heterozygote cells by double label alkaline elution. The single-strand break in these three different lines of cells after 10 Gy X-radiation is about the same, but there is significantly different from the contral. After 37℃ incubation, the single-strand break of these three lines cells repaired accordingly with the time 15,30, 60 minutes. There are almost repaired within 60 minutes. The single-st-rand break in three different lines of cells are partly repaired after 37℃ incubation when 5×10,-4, mol/L aphidicolin was added.