Human diploid fibroblasts lose the capacity to proliferate and enter a state termed replicative senescence after a finite number of cell divisions in culture.When treated with sub-lethal concentrations of H₂O₂, pre-senescent human fibroblasts enter long-term growth arrest resembling replicative senescence. To understand the molecular basis for the H₂O₂-induced growth arrest, we determined the cell cycle distribution, levels of p53 tumour suppressor and p21 cyclin-dependent kinase inhibitor proteins, and the status of Rb phosphorylation in H₂O₂-treated cells. A 2-h pulse of H₂O₂ arrested the growth of IMR-90 fetal lung fibroblasts for at least 15 days. The arrested cells showed a G₁ DNA content. The level of p53 protein increased 2- to 3-fold within 1.5 h after H₂O₂ exposure but returned to the control level by 48 h. The induction of p53 protein was dose dependent, beginning at 50–75 µM and reaching a maximum at 100–250 µM. The induction of p53 did not appear to correlate with the level of DNA damage as measured by the formation of 8-oxo-2´-deoxyguanosine in DNA. The level of p21 protein increased about 18 h after H₂O₂ exposure and remained elevated for at least 21 days. During this period, Rb remained underphosphorylated. The induction of p53 by H₂O₂ was abolished by the iron chelator deferoxamine and the protein synthesis inhibitor cycloheximide. The human papillomavirus protein E6, when introduced into the cells, abolished the induction of p53, reduced the induction of p21 to a minimal level and allowed Rb phosphorylation and entry of the cells into S-phase. The human papillomavirus protein E7 reduced the overall level of Rb and also abolished H₂O₂-induced G₁ arrest. Inactivating G₁ arrest by E6, E7 or both did not restore the replicative ability of H₂O₂-treated cells. Thus H₂O₂-treated cells show a transient elevation of p53, high level of p21, lack of Rb phosphorylation, G₁ arrest and inability to replicate when G₁ arrest is inactivated.