Objectives: NK cells play a key role in the early defense against viral infections and tumors. In C57BL/6 mice NK cells express Ly49H activating receptor which specifically recognizes virally encoded protein m157 on the surface of cells infected with mouse cytomegalovirus (MCMV). The activation of NK cells through Ly49H-m157 interaction results in the early restriction of virus replication. The preliminary data from our study showed that the absence of an early NK cell activation through Ly49H-m157 interaction results in higher activation of CD8+ T cells in acute phase of the MCMV infection. Guided by these findings, our goal was to determine the mechanism that regulates the development and intensity of the specific CD8+ T-cell response concerning the lack of early activation of NK cells via Ly49H receptor. Materials and methods: Ly49H+ and Ly49H– mice were infected with wild-type virus (WT MCMV) or mutant virus lacking m157 gene (m157 MCMV). The contribution of individual lymphocyte subsets in the control of MCMV infection was determined after depletion of these subsets with monoclonal antibodies, and by comparing the growth of the virus in different tissues of MCMV infected mice by standard plaque assay. Phenotypic and functional status of immune cells during MCMV infection was tested by flow cytometry and by determination of cytokine levels in the sera of infected mice. Results: This study showed that the strength of CD8+ T-cell response to MCMV differs in Ly49H+ and Ly49H– mice. Our results in C57BL/6 mice demonstrated that CD8+ T-cell response in the first week of MCMV infection is determined by the engagement of Ly49H receptor and the capacity of NK cells to control virus replication during the first two days. Infection with WT MCMV resulted in strong NK cell activation that restricted virus replication, and consequently impaired MCMV-specific CD8+ T-cell response. The importance of specific activation of NK cells through Ly49H receptor in the regulation of intensity of CD8+ T-cell response was confirmed by using mutant virus m157 MCMV. The inability of NK cells to restrict the replication of m157 MCMV resulted in high viral titers in organs, increased frequency of infected conventional dendritic cells and high levels of proinflammatory cytokines, and all these factors contributed to the development of strong CD8+ T-cell response. By using model of immunodeficient mice, we have shown that, besides specific activation through Ly49H-m157 interaction, the immunoregulatory role of NK cells also depends on perforin mediated cytolytic mechanism. Conclusion: The results of this study further expanded our knowledge about immunoregulatory role of NK cells in the modulation of CD8+ T-cell response to MCMV. The specific activation of NK cells through Ly49H-m157 interaction and perforin mediated cytolytic mechanism were shown to be the key factors in determining the kinetics and intensity of MCMV-specific CD8+ T-cell response during acute infection.