SUMMARY

The influence of ionising radiations on ageing is still controversial. Since Hayflick established the concept hat diploid cells have finite lifespan in vitro, human diploid fibroblast (HDF) cultures have been recognised as a potent experimental model for cyto-gerontological investigations. In this study HDF cultures in phase II were exposed to acute irradiation with either X-rays or fast neutrons. The replicative potentials and labelling indices with [3H]thymidine were measured post irradiation until the cultures ceased growth in phase III. Cell mortality was measured by cloning. The apparent loss in replica-tive potential of irradiated mass cultures was wholly attributable to the loss of viable clonogenic ells. The current concept of precocious clonal senescence in vitro as a late effect of irradiation in clonogenic survivors is not supported by the present experiments. Instead, our results suggest hat exposure to a single dose of ionising radiations either causes total replicative incapacitation (killing) of HDF cells and their progeny early after irradiation or leaves their replicative potentials unperturbed