An ELISA was used to study long-term immunity and immunization responses to tetanus toxoid in 48 bone marrow transplant recipients. Among patients who were seropositive to tetanus before transplant, 51% had lost their seropositivity 1 year later. All patients who were not reimmunized with tetanus toxoid were seronegative 2 years after transplant. All patients who were seronegative before transplant remained seronegative 1 year later regardless of the donor's serologic status. There was no difference in the ability to remain seropositive to tetanus toxoid between patients with and without chronic graft-versus-host disease. Of 21 patients immunized with one dose of tetanus toxoid 1 year after transplant, 14 were seronegative at the time of immunization (response rate, 64%). At 1 year after immunization, 7 remained seropositive. Ten patients were reimmunized with two doses of tetanus toxoid. All responded and 90% remained seropositive 1 year later. When 21 patients were primarily immunized with three doses of tetanus toxoid, all patients seronegative at immunization responded and all tested patients remained seropositive 2 years later. The immunization responses were significantly superior in patients receiving three doses compared with those who received one. Reimmunization with tetanus toxoid of long-term survivors after marrow transplant seems necessary. A three-dose immunization schedule is recommended to obtain an adequate immune response.