Cytotoxic activity attributable to natural killer (NK) cells was augmented in the spleens of mice infected with murine cytomegalovirus (MCMV). Stimulation was observed as early as 10 hr after virus administration but was dependent upon the dose used, and in particular, the host genotype. Effector cells were capable of killing a range of tumor cell targets as well as normal syngeneic thymocytes, and were induced in nude (nu/nu) and heterozygous (nu/+) mice. A significant correlation existed between resistance to the lethal effects of MCMV infection and the degree of NK cell augmentation by this virus in 10 of 11 strains examined. Non-H-2 related differences in activity were observed, but a clear association between high NK cell responses and the possession of a particular H-2 haplotype was not apparent. Beige mutant C57BL/6J mice, previously reported as defective in NK cell function were susceptible to MCMV and expressed low levels of cytotoxicity during infection. Thus, the genetically controlled stimulation of NK cells in the early stages of infection with MCMV may contribute to the genotype related patterns of resistance seen with this virus.