The antiviral effect of natural human interferon-beta (HuIFN-beta) against human cytomegalovirus (CMV) was evaluated in human embryonic lung fibroblasts (HEL). Natural HuIFN-beta, like other HuIFNs, inhibited the replication of CMV. Pretreatment of the cells with natural HuIFN-beta inhibited the appearance of immediate-early antigen (IEA) or pre-early nuclear antigen (PENA) as well as the production of infectious CMV. After a single treatment with natural HuIFN-beta, intracellular 2', 5'-oligoadenylate (2-5A) synthetase activity was induced and maintained at a high level for several days. The anti-CMV effect of natural HuIFN-beta correlated with the intracellular 2-5A synthetase activity.