ATAXIA telangiectasia (AT) is an autosomal recessive defect in man showing among the clinical features¹ : cerebellar ataxia, telangiectasia, IgA deficiency, an enhanced frequency of malignancy and an enhanced level of spontaneous chromosome instability². There have also been reports of increased sensitivity to X rays after radiotherapy^3–5 and increased chromosome aberrations induced by ionising radiation in leukocyte cultures from AT patients^6,7. We report here cell survival experiments which indicate that the clinically observed enhanced sensitivity of AT patients to ionising radiation is manifest at the cellular level.