Expression of Simian Virus 40 (SV40) T antigen in human dermal fibroblasts over-rides the normal controls on cell division leading to changes in cellular proliferation and life span. These changes are accompanied by other changes in cell morphology, expression of cell specific functions, and altered cell-cell interactions. In this study, we have examined the effects of different amounts of T antigen on cell cycle progression, life span and morphology in human dermal fibroblasts and demonstrated T antigen to be a concentration dependent regulator of the cell cycle. Using a novel, metal inducible episomal expression vector (p735.6) which produces low basal levels of protein but high (greater than 100-fold) levels of induction, we have compared the effects of low and high levels of T antigen expression in a precrisis and immortalised human line (1BRMT1). The presence of inducing agent led to maximal levels of T antigen expression and resulted in cultures with a high rate of proliferation, an extended in vitro life span, a loss of contact inhibition of growth and a morphology characteristic of SV40-transformed cells. In the absence of inducing agent, read-through of the T antigen gene resulted in low but detectable levels of protein. The reduction in T antigen levels was accompanied by a 50% or greater reduction in the proliferative rate and restoration of cell morphology and contact inhibition similar to that found in non-transfected cells. The results presented here demonstrate that low amounts of T antigen are sufficient to maintain cell viability and prevent the re-expression of the senescent phenotype seen in the absence of T antigen. Similarly, the ability of T antigen to extend the in vitro life span is not dependent on high level expression of T antigen. In contrast, the rate of proliferation of human cells as well as the cell morphology and contact inhibition are dependent on the amount of T antigen present. Many of the cellular effects can be minimised or reversed by reducing T antigen expression.