In this paper we review the current data on the role of potentially lethal damage (PLD) recovery in human tumour cell lines, both in vitro and in vivo. In the case of cell lines studied in vitro, the mean recovery ratios found were higher for cells derived from tumours of low curability (glioblastoma, hypernephroma, osteosarcoma, melanoma) than for cells derived from tumours of high curability (breast carcinoma, neuroblastoma). Experiments were performed in vivo only with tumours of low and intermediate curability (melanoma, adenocarcinoma of the colon, pancreatic tumour). Although fragmentary and obtained only with established cell lines, these results argue in favour of the occurrence of PLD repair in human tumour, the amplitude of this repair being, in certain cases, sufficient to explain the incurability of a tumour by radiation therapy.