Latent Japanese encephalitis virus (JEV) infection was shown in inapparently congenitally infected Swiss albino mice after their mothers had been given JEV intraperitoneally during pregnancy. Only one of 37 (2.7%) of the baby mice showed persistence of infectious virus at 5 weeks of age. Reactivation of JEV in Swiss albino mice was demonstrated by stimulation with allogeneic spleen cells from Parks strain mice at 21 weeks of age; reactivation was demonstrated in 41% of the inapparently infected mice. The spleen cells of congenitally infected mice had depressed [3H]thymidine uptake following stimulation with concanavalin A, and depressed ability to induce a graft-versus-host response.