The effect of prophylactic intravenous administration of a cytomegalovirus (CMV) hyperimmune globulin with a high titer of neutralizing antibodies plus oral acyclovir was studied in 93 consecutive bone marrow transplant recipients. In spite of receiving blood products unscreened for CMV only six patients developed CMV infections during the time they received passive immunization. Five patients reactivated virus after hyperimmune globulin infusions were stopped; four of them suffered from chronic graft-versus-host disease (GVHD) Among the patients suffering from acute GVHD grade III/IV and/or chronic GVHD the incidence of CMV infection (10/38) was significantly higher than among those with no or milder forms of GVHD (1/55) (p less than 0.01). Only three patients suffered from symptomatic CMV infections; two with gastrointestinal manifestations and one with fatal CMV pneumonia. Thus CMV prophylaxis as used here proved highly effective in combating one of the major difficulties encountered in BMT.