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The immediate-early 1 (IE1) and IE2 proteins encoded by the major immediate-early (MIE) transcription unit of cytomegaloviruses are thought to play key roles in the switch between latent- and lytic-cycle infection. Whilst IE2 is essential for triggering the lytic cycle, the exact roles of IE1 have not been resolved. An MIE-exon 4-deleted rat cytomegalovirus (DeltaIE1) failed to synthesize the IE1 protein and did not disperse promyelocytic leukaemia bodies early post-infection, but was still capable of normal replication in fibroblast cell culture. However, DeltaIE1 had a diminished ability to infect salivary glands persistently in vivo and to reactivate from spleen explant cultures ex vivo. Quantification of viral genomes in spleens of infected animals revealed a reduced amount of DeltaIE1 virus produced during acute infection, suggesting a role for IE1 as a regulator in establishing a chronic or persistent infection, rather than in influencing the latency or reactivation processes more directly.