In a double-blind placebo-controlled study, the value of prophylactic anti-CMV immunoglobulin administration was evaluated in 39 kidney transplant recipients treated for rejection with rabbit antithymocyte globulin. Passive immunization completely prevented CMV-related death, although it did not reduce the incidence of CMV isolation, viremia, or disease.The effect of passive immunization was exclusively observed in CMV-seronegative recipients of a CMV-seropositive kidney donor. It could be demonstrated even when instituted when antirejection therapy was started. Seropositive recipients did not benefit from immunoglobulin treatment. Moreover, CMV-seronegative recipients of a kidney from a seronegative donor were not at risk for CMV infection at all. Therefore passive immunization should be restricted to seronegative recipients of seropositive allograft donors treated for rejection.