Propagation of CELO virus employing confluent monolayers of chicken enbryo fibroblasts (CEF) yielded virus titers one to two logs lower than those from confluent chicken kidney (CK) cells. An enhancement of virus production in CEF as measured by plaque formation was obtainedby infectng cultures in the growing non confluent state. Measurements of 3H-thymidine incorporation revealed a positive correlation between the DNA synthesis of CEF cultures at the time of inoculation and the amount of progeny virus, whereas in the CK-CELO-system no such relation was observed. Requirement of replicative fibroblasts for CELO multiplication was also demonstrated by comparison of virus replication in synchronized stationary and serum stimulated CEF cells. In stationary CEF cells arrested in the G1 phase of the cell replication cycle by serum deprivation and infected withe CELO virs, no cytopathic effect could be observed, and only very low amounts of virus were produced. But 24 hours after release of these cells for growth by serum stimulation a logarithmic rate of virus multiplication and a complete CPE occurred. Infection of synchronized CEF cultures at different stages of the cell cycle revealed that CELO multiplication was correlated with the S phase of the infected cell. In synchronized CELO infected CEF cultures viral DNA synthesis started 12 to 14 hours after growth stimulation when cells were near the end of the S phase. In contrast, no viral DNA synthesis could be measured in growth arrested CELO infected CEF cells, when cellular DNA synthesis was low. Therefore not only production of infectious virus but also viral DNA synthesis is correlated with events during the S phase of the infected CEF cell.