Wild type p53 can induce cell cycle arrest at specific points in the cell cycle, in particular G1/S, an ability lost by most p53 mutants. We have previously reported that p53 mutant genes can rescue REF52 cells from ras-induced growth arrest and that over expression of wild type p53 inhibits cell growth in these cells. In this paper we examined whether p53 can also induce cell cycle arrest at the G2/M boundary of the cell cycle. To accomplish this we used the REF52 cell line and the temperature sensitive p53val135 mutant allele. Cells were enriched in the late G1 and early S phases before the temperature shift. REF52 cells expressing mutant-p53val135 alone with an activated H-ras gene arrest primarily at the G1/S and G2/M parts of the cell cycle at the restrictive temperature, as determined by flow cytometry analysis. These results suggest that the anti-proliferative activity of p53 may be involved in regulation of the cell cycle at the G2/M restriction point as well as transit through G1/S and initiation of DNA synthesis.