Serotherapy treats a transplant hurdle Cytomegalovirus (CMV) infection and reactivation are common and potentially fatal complications after bone marrow or hematopoietic stem cell transplantation (BMT). Martins et al. developed faithful preclinical murine models of CMV reactivation following BMT and found that humoral immunity can prevent this process (see the Perspective by Alegre). After BMT, antiviral antibodies that would have kept CMV at bay dwindle because host plasma cells are ablated and the donor B cell pool reconstitutes poorly. CMV reactivation was prevented by transferring antibody-containing immune serum. Such a therapeutic strategy would avoid some limitations of cellular therapies for BMT patients. Science , this issue p. 288 ; see also p. 232