Immunity to tetanus toxoid and polioviruses was studied in 34 (27 allografted, 7 autografted) children who underwent bone marrow transplantation (BMT). At a median time of 3 years after BMT, only one recipient was seronegative for tetanus toxoid. On the contrary 73% of children were seronegative for at least one of the three poliovirus types and 30% for all virus types. Undetectable antibody titers were more frequently found against type 3 than the other two types. We recommend that reimmunizations of children after BMT be based on serologic tests for antibody titers.