It was shown earlier that in cultures of contact-inhibited mouse kidney cells infection with polyoma virus induces replication of the chromosomal DNA. In this paper we present evidence, based on analyses of isolated chromatin, that, parallel with and as a consequence of the virus-induced synthesis of cellular DNA, the chromosomal proteins are synthesized in the proportions characteristic of normal chromosome replication. These results are compatible with the hypothesis that polyoma virus activates (or derepresses) a regulatory system of the host cell which controls initiation of chromosome replication.