Mammalian cells in culture normally enter a state of quiescence during G1 following suppression of cell cycle progression by senescence, contact inhibition or terminal differentiation signals. We find that mammalian fibroblasts enter cell cycle stasis at the onset of S phase upon release from prolonged arrest with the inhibitors of DNA replication, hydroxyurea or aphidicolin. During arrest typical S phase markers remain present, and G0/G1 inhibitory signals such as p21(WAF1) and p27 are absent. Cell cycle stasis occurs in T-antigen transformed cells, indicating that p53 and pRB inhibitory circuits are not involved. While no DNA replication is evident in arrested cells, nuclei isolated from these cells retain measurable competence for in vitro replication. MCM proteins are required to license replication origins, and are put in place in nuclei in G1 and excluded from chromatin by the end of replication to prevent rereplication of the genome. Strikingly, MCM proteins are strongly depleted from chromatin during prolonged S phase arrest, and their loss may underlie the observed cell cycle arrest. S phase stasis may thus be a 'trap' in which cells otherwise competent for S phase have lost a key component required for replication and thus can neither go forward nor retreat to G1 status.