It has been shown that in cultures of mouse cells the satellite simple-sequence DNA is replicated late in S phase1–3 and similarly in secondary fibroblasts from mouse embryos synchronized by serum shift4. By contrast, it has been reported that when confluent baby mouse kidney (BMK) cells are infected with polyoma virus, the satellite DNA replicates before the remainder of the host cell DNA5. Contact-inhibited cells are thought to be arrested at the end of the G1 phase6, but after infection with polyoma virus these contact-inhibited BMK cells are stimulated to complete another cycle of DNA replication7. Dulbecco et al.7 showed that the majority of this host cell DNA synthesis began 16 h after viral infection. Smith5 has reported that the DNA synthesized after this time is almost exclusively main-band DNA, but that the light satellite DNA is replicated earlier, between 12 and 16 h after infection.