There is a progressive decline in replicative capacity with increasing age as expressed in terms of percentage labelled nuclei with 3H-thymidine and altered saturation density at confluency. This expression of ageing in vitro is seen in three different lines of human embryo diploid fibroblasts, although the pattern and rate of decline is different in each case. Generalization about in vitro ageing from studies with one cell line should therefore be made with care or avoided. There was an increase in total cellular RNA content as cultures aged which was more pronounced as cells entered the senescent or terminal phase of their lifespan. This increase appeared to be accompanied by a slightly elevated uptake and incorporation of 3H-uridine per cell. Template activity of isolated nuclei was markedly reduced in very late passage or phase III cells, but did not show a progressive decline with increasing age. These studies show that there is a reduced replicative potential which is not accompanied by a detectable decline in transcription, and suggest that the altered template activity should be regarded as an effect of ageing in vitro.