Infection is a common problem for bone marrow transplant (BMT) recipients during the period of neutropenia that immediately follows the procedure. Gram-negative infections present a particular hazard in these immunocompromised hosts. To augment host defenses against one such pathogen, Pseudomonas aeruginosa, we immunized bone marrow transplant donors and/or recipients with a polyvalent O-polysaccharide- toxin A conjugate vaccine. When either donor or recipient alone was vaccinated before transplant, no increase in specific antibody titers to any of the vaccine components was observed in the recipient. However, when both donor and recipient were vaccinated before transplant, increases in antibody titers to all polysaccharide components occurred to levels shown to be protective in animal models of gram-negative sepsis. Specific antibodies were primarily of the IgG1 and IgG2 subclass even though IgG2 subclass deficiency is common after BMT. The requirement for both donor and recipient immunization reflects the need for primed donor B lymphocytes in the marrow inoculum to be transferred into an antigen-containing environment so that maximum B- cell proliferation and antibody secretion can occur. Adoptive transfer of antibody responses to Pseudomonas aeruginosa and other common bacterial pathogens has the potential to reduce infection-related morbidity and mortality after allogeneic bone marrow transplantation.