The increase in autofluorescence (AF) of human skin fibroblasts during their in vitro ageing and growth inhibition was investigated by means of flow cytophotometry. The cellular AF of in vitro ageing cultures increased while the relative number of (3H)-thymidine incorporating cells decreased. Therefore, the rate of accumulation of cellular AF during in vitro ageing of the cultures is inversely related to the proliferation rate of the culture. The rates of increase of AF varied widely among the cell strains, being the highest in cells from patients with Werner's syndrome. Upon growth inhibition in a confluent culture the net rates of increase of cellular AF were found to vary widely among the cell strains. The respective net rates of increase of AF of the cells from patients with Werner's syndrome and the Spielmeyer-Vogt syndrome were within the range covered by the normal cell strains. The ultrastructure of the bright AF cells from patients with Werner's syndrome and the Spielmeyer-Vogt syndrome differed from the ultrastructure of AF cells from control persons with regard to the morphology of their residual bodies, those from the patients contained more multilamellar and multivesicular structures. In sorted non-AF cells vitamin E was found to completely inhibit the accumulation of AF without affecting the formation of 'residual bodies'. We infer that cellular AF is caused by lipid peroxidative reactions and that the accumulation of AF is due to a decrease in cellular proliferation rate.