Live guinea pig cytomegalovirus (CMV) vaccine was prepared after 11 serial passages in tissue culture; noninfectious envelope antigen vaccine was prepared by n-octyl glucoside treatment of CMV-derived dense bodies and virions. Hartley strain guinea pigs immunized with either vaccine were compared with guinea pigs inoculated with virulent, salivary gland-passaged CMV (approximating natural infection), with passively immunized animals, and with nonimmune controls. All vaccinated animals had neutralizing antibodies to CMV. After challenge with virulent CMV, animals previously inoculated with either tissue culture-passaged or virulent CMV were protected against acute viremia and death; pregnant animals previously inoculated with live CMV vaccine had lower incidences of viremia and generalized maternal and fetal infection. Envelope antigen-vaccinated and passively immunized pregnant animals showed acute viremia after similar challenge with virulent virus; however, infection was less generalized than that in control animals, and CMV was not isolated from the fetuses of these vaccinated mothers.