Transfer of specific immunity was investigated in a group of 28 paediatric and adult leukaemia patients during the first 100 d after allogeneic bone marrow transplantation (BMT). These patients and/or their donors were immunized 7-13 d before transplantation with the recall antigen tetanus toxoid (TT) and the neo-antigen Helix pomatia haemocyanin (HPH). The recipients were booster immunized with both antigens at day 42 after transplantation. Transfer of a primary IgM and IgG response to HPH was successful in most paediatric and adult patients, but transfer of a secondary response to TT was established in only a few paediatric recipients. After booster immunization at day 42 most paediatric recipients responded with a rise in serum antibody titre to HPH as opposed to only two of 18 adult recipients. This incapability of the adult recipients to mount a secondary immune response may be related to their conditioning regimen which included Campath-IG in vivo. The results from this study indicate that transfer of immunity against recall- and neo-antigens is possible. However, the establishment of long-term memory may be affected by the regimen used to condition the graft recipient.