Improvements in tissue culture techniques and growth media have made it possible to culture a range of cells of human origin, both normal and malignant. The most recent addition to the list are endothelial cells from umbilical cord veins. Interesting results in radiosensitivity studies of these human cells have been obtained, some of which may have implications in radiation therapy. (i) Repair of potentially lethal damage (PLDR) has been observed in all cell lines investigated; cells of normal origin repair PLD at least as well as malignant cells, which makes clinical trials of PLDR inhibitors of doubtful usefulness. (ii) No apparent correlation can be made between the extent of PLDR and the traditional radioresponsiveness of a particular tumor type. Indeed, if anything, it could appear to have an inverse correlation since the most resistant tumor cells show the smallest amount of PLD repair. (iii) Dose-rate effects appear to be better predictors of radiosensitivity than PLDR capacity. (iv) Sublethal damage repair, manifest by a dose-rate effect, has also been observed in all human cell lines tested. Cells of normal tissue origin, including fibroblasts and endothelial cells, exhibit a dose-rate effect that is intermediate between that for cells from traditionally resistant tumors (melanoma and osteosarcoma) and cells from more sensitive tumors (neuroblastoma and breast).