After infection in utero or at birth with a cell culture adapted strain of mouse cytomegalovirus (MCMV), several mouse strains developed a latent virus infection in the presence of specific antiviral antibodies. Up to 5 mo after infection, MCMV could be activated and recovered from spleen lymphocytes of the infected animals that were co-cultivated with histoincompatible (H-2 foreign) mouse embryo cells from uninfected animals. In contrast, co-cultivation of lymphoid cells from infected mice with mouse embryo cells from syngeneic, histocompatible (H-2 similar) donors did not activate MCMV. Similarly, MCMV was not recovered from sonicated lymphoid cells. Virus was activated by treating viable lymphoid cells with lipopolysaccharide, a B-cell mitogen, but was not activated by a variety of other mitogens such as phytohemagglutinin, concanavalin A, or pokeweed mitogen. Subsequent purification of lymphoid cells from the infected mice by a variety of techniques indicated that MCMV was harbored in the B-lymphocyte population.