Antiviral humoral reactivity was studied after bone marrow transplantation. Antiviral IgG--reflecting the presence of functional B cells--was found for a substantial period after transplantation. However, most patients unexpectedly stopped producing antibody after an extended period of observation (i.e., for more than three months and up to as long as 12 months). This cessation was noted whether the viral reactivity involved was of donor or recipient origin. The transferred viral antibody-producing cells thus usually persisted for only a moderate length of time and seemed to have a finite life span. The findings indicate that donor and recipient memory cells are susceptible to the conditioning regimen or to the treatment that follows transplantation. In a few cases transferred immunity of long duration was seen; this finding indicated the occasional engraftment of memory cells.