The sources of cytomegalovirus (CMV) infection in kidney transplant recipients include reactivation of latent endogenous virus in the recipient or reactivation of latent virus in donated blood or kidney. In the present study, kidneys from mice latently infected with one strain of murine CMV were transplanted into either uninfected recipients or recipients latently infected with a different strain of murine CMV; the recipients were immunosuppressed, subsequently were cultured for murine CMV, and the infecting strain was characterized. The results show that reactivation of latent murine CMV from the donated kidney can be the source of active infection in previously uninfected recipients. When the recipient had been previously infected, however, reactivation of the endogenous recipient strain of murine CMV was the source of active infection in 10 of 12 instances. At no time were both exogenous and endogenous strains of virus reactivated simultaneously. These studies indicate that donor kidney may be the source of latent virus in the uninfected recipient but that endogenous virus predominates in previously infected recipients.