Sublethal oxidative stresses increase the proportions of human fibroblasts positive for senescence associated beta-galactosidase activity and accelerate the transition in the fibroblast morphotypes characterising fibroblast ageing. Stimulation of fibroblasts with TNF-alpha or IL-1alpha transiently increases the production of reactive oxygen species (ROS) in human fibroblasts. Here we propose that repeated stimulation of WI-38 fibroblasts with TNF-alpha or IL-1alpha can generate enough ROS to accelerate the transition in the fibroblast morphotypes and increase the proportion of cells positive for senescence associated beta-galactosidase activity. The involvement of ROS is suggested by experiments where the stimulation of fibroblasts with TNF-alpha or IL-1alpha are performed in the presence of N-acetylcysteine which increases the intracellular antioxidant potential. It is proposed that the decrease in the proportions of morphotypes I and II, and the increase in the proportions of morphotypes III to VI observed after successive stimulation with TNF-alpha or IL1-alpha is attributed to an increased ROS production occurring during the stimulation.