Cytomegalovirus (CMV) establishes lifelong chronic infection in its host with mostly asymptomatic or only mild disease, but under immunosuppressive conditions the virus can reactivate and infection can cause life-threatening disease. CMV has evolved several mechanisms to escape from host's immunity, allowing persistence of the virus. Until now, it remains elusive whether regulatory T cells (Tregs) have an impact on insufficient host immune response against the virus in this context. In the present study, we provide evidence that CD4(+)Foxp3(+) naturally occurring Tregs (nTregs) as well as CD4(+)Foxp3(-)IL-10(+)-induced Tregs (iTregs) interfere with an effective anti-mouse CMV (mCMV) immune response. Depletion of Foxp3(+) Tregs by using DEREG mice resulted in enhanced T-cell activation as measured by the expression of CD62L, granzyme B and interferon (IFN)-gamma and was associated with reduced viral titers in salivary glands, the organ where mCMV mainly persists. Moreover, we identified CD4(+)Foxp3(-) T cells to produce elevated levels of the immunosuppressive cytokine IL-10 at early time points during mCMV infection. Analysis of T-cell activation and viral replication in mCMV- infected IL-10(flox/flox) x CD4-cre mice and IL-10(flox/flox) x FIC mice revealed that T-cell-specific inactivation of IL-10, but not Foxp3(+) Treg-specific IL-10 ablation alone, resulted in elevated IFN-gamma production by T cells associated with a significant decrease in viral loads in salivary glands. Thus, our data illustrate a crucial role for CD4(+)Foxp3(+) nTregs as well as IL-10-producing CD4(+)Foxp3(-) iTregs in the regulation of appropriate T-cell responses and viral clearance during mCMV infection.