Ataxia telangiectasia (AT) is an autosomal recessive genetic disease. It was proved that a SV 40-transformed AT cells line, AT 5 BIVA, from the skin fibroblasts of an AT patient, was hypersensitive to ionizing radiation as well as to bleomycin and was radioresistant to DNA synthesis inhibition. Chromosome analysis had shown all fourteen AT 5 BIVA cells analysed had one of the number 11 chromosomes lost or rearranged, enhancement of transcient radioresistance by DNA mediated gene transfer was found, either with HeLaS 3 genomic DNA or with its restrction endonuclease digested fragments. The reason for this pheonmenum was discussed.