The immunological response of spleen cells was examined during murine cytomegalovirus (MCMV) infection of germfree (GF) mice. GF mice were more susceptible to MCMV infection than specific pathogen-free (SPF) mice. Recovery from the acute phase of MCMV infection was delayed in GF mice as compared to SPF mice. During MCMV infection, the immune response of GF mice, measured in terms of mitogen and of antibody response to sheep erythrocytes, was more markedly depressed and the recovery from the depression was more delayed than in SPF mice. In MCMV-infected mice, natural killer (NK) cell activity was increased by day 1 and 2 post-infection, with similar kinetics in GF and SPF mice, suggesting that NK cells may be exerting the most potent antiviral effect during the early stage of MCMV infection.