The repair of DNA single-strand breaks induced by X-rays or bleomycin was investigated in diploid fibroblasts isolated from normal individuals and from patients with ataxia telangiectasia by the technique of alkaline elution. No difference was observed between these cell strains in the rate of rejoining of DNA strand breaks induced by low or moderate doses of X-rays or by treatment with bleomycin. Owing to the sensitivity of the technique employed, the possibility that ataxia telangiectasia cells are deficient in DNA single-strand break repair appears unlikely.