The highly related acetyltransferases, p300 and CREB-binding protein (CBP) are coactivators of signal-responsive transcriptional activation. In addition, recent evidence suggests that p300/CBP also interacts directly with complexes that mediate DNA replication and repair. In this report, we show that loss of p300/CBP in mammalian cells results in a defect in the cell cycle arrest induced by stalled DNA replication. We demonstrate that complexes containing p300/CBP and ATR can be detected in mammalian cells, and that the downstream kinase CHK1 fails to be phosphorylated in response to stalled DNA replication in cells that lack p300/CBP. These observations broaden the roles for the p300/CBP acetyltransferases to include the modulation of chromatin structure and function during DNA metabolic events as well as for transcription.