DR L. K. BLEYMAN1 need not be concerned that our experiments on clonal senescence of human cells in culture are being carried out in ignorance of earlier studies of such ageing in protozoa. Indeed the paper he cites2 refers to one important study carried out with protozoa. The experiments with human cells are derived directly from earlier work with simple eukaryotic organisms such as fungi. When the first results were being prepared for publication3, reference was made to related studies with ciliated protozoa and a copy was sent to Professor T. M. Sonneborn for comment. In subsequent correspondence with him it became clear that it would not be possible (in a paper limited in length by the editor of Nature) to do justice to the many important observations on the complex interaction between nuclear and cytoplasmic degenerative changes during ageing in these organisms. It would be most valuable if Dr Bleyman could now be persuaded to publish a review of these experiments in the light of subsequent biochemical studies on the mechanism of clonal senescence in other organisms.