Until recently, the ability of ARF (human p14 ARF, murine p19 ARF) tumour-suppressor protein, encoded by the INK4A/ARF locus, to inhibit cell growth in response to various stimuli was related to its ability to stabilize p53 through the so-called ARF/MDM2/p53 pathway. However, recent data have demonstrated that ARF is not implicated in this unique p53-dependent pathway. By use of transient and stable expression, we show here that human p14 ARF inhibits the growth of human tumoral cells lacking functional p53 by inducing a transient G 2 arrest and subsequently apoptosis. This p14 ARF–induced G 2 arrest was correlated with inhibition of CDC2 activity, inactivation of CDC25C phosphatase and induction of the CDK inhibitor p21 WAFI. Apoptosis was demonstrated using Hoechst 33352 staining, proteolytic activation of caspase-3 and PARP cleavage. Similar results were obtained in experiments with cells synchronized by hydroxyurea block. Importantly, we were able to reproduce these effects ‘in vivo’by showing that p14 ARF inhibits the growth of p53 nullizygous human tumours in nude mice and induces the regression of p53−/− established tumours. In these experiments, tumoral regression was associated with inhibition of cell proliferation as well as induction of apoptosis confirming the data obtained in cell lines.