Mammalian viruses often use components of the host's cellular DNA replication machinery to carry out replication of their genomes, which enables these viruses to be used as tools for characterizing factors that are involved in cellular DNA replication. The human papillomavirus (HPV) E1 protein is essential for replication of the virus DNA(1-3). Here we identify the cellular factor that participates in viral DNA replication by using a two-hybrid assay(4) in the yeast Saccharomyces cerevisiae and E1 protein as bait. Using this assay, we isolated Ini1/hSNF5 (ref. 5), a component of the SWI/SNF complex which facilitates transcription by altering the structure of chromatin(6). In vitro binding and immunoprecipitation confirmed that E1 interacts directly with Ini1/hSNF5. Transient DNA-replication assay revealed that HPV DNA replication is stimulated in a dose-dependent manner by addition of Ini1/hSNF5, and that Ini1/hSNF5 antisense RNA blocks the replication of HPV DNA. Amino-acid substitution at residues that are conserved among E1 proteins prevented the E1-Ini1/hSNF5 interaction and reduced DNA replication of HPV in vivo. Our results indicate that Ini1/hSNF5 is required for the efficient replication of papillomavirus DNA and is therefore needed, either alone or in complex with SWI/SNF complex, for mammalian DNA replication as well.