p53 is a cellular phosphoprotein that is present at elevated concentrations in cells transformed by different agents1-5. p53 complementary DNA expression-constructs immortalize primary cells in vitro6,7 and co-operate with an activated ras oncogene in malignant transformation 6,8,9. Several reports have implicated p53 in mammalian cell cycle control and specifically with events occurring at the G0-G 1 boundary10,11. p53 forms specific complexes with simian virus 40 (SV40) large-T antigen1,2, and such complexes are found associated with both replicating and mature SV40 DNA in lytically infected cells12. In an accompanying paper Gannon and Lane13 report that in in vitro plate-binding assays, mouse p53 can displace polymerase α from complex with T-antigen. We have examined the in vivo consequences of expressing wild-type and mutant p53 proteins from other species in SV40-transformed monkey cells. We report here that expression of mouse p53 results in a substantial and selective inhibition of SV40 origin-dependent DNA replication. In addition to any function in the G0-G1 transition, the data presented suggest that p53 may affect directly the initiation or maintenance of replicative DNA synthesis.