The p16(INK4a) cyclin-dependent kinase inhibitor(1) is implicated in replicative senescence, the state of permanent growth arrest provoked by cumulative cell divisions or as a response to constitutive Ras-Raf-MEK signalling in somatic cells(2-8). Some contribution to senescence presumably underlies the importance of p16(INK4a) as a tumour suppressor(9) but the mechanisms regulating its expression in these different contexts remain unknown. Here we demonstrate a role for the Ets1 and Ets2 transcription factors(10) based on their ability to activate the p16(INK4a) promoter through an ETS-binding site and their patterns of expression during the lifespan of human diploid fibroblasts. The induction of p16(INK4a) by Ets2, which is abundant in young human diploid fibroblasts, is potentiated by signalling through the Ras-Raf-MEK kinase cascade and inhibited by a direct interaction with the helix-loop-helix protein Id1 (ref. 11). In senescent cells, where the Ets2 levels and MEK signalling decline, the marked increase in p16(INK4a) expression is consistent with the reciprocal reduction of Id1 and accumulation of Ets1.