A strategy for the production of human antibodies to tetanus toxoid (TT) is described. Human peripheral blood lymphocytes (PBL) were injected into severe combined immunodeficient (SCID) mice (termed hu-PBL-SCID) and the mice subsequently immunised with purified TT. By using low immunising doses of antigen, PHA activated PBL and PBL from donors who were recently immunised with TT, we established ongoing antibody responses to TT with specific recall IgG responses of up to 22 IU ml-1 in the murine plasma, which was greater than that in the donors' serum. Total IgG concentrations of up to 6 mg ml-1 were detected over a 32 week period. Lower levels of IgM and IgM anti-TT were also detected over this time. Large cellular infiltrations of human CD45+ and CD20+ cells were detected by immunocytochemistry in the mesenteric membranes, mesenteric lymph nodes and the pancreas 5 weeks after PBL were engrafted into a SCID mouse. Human cells were also observed in the lungs, liver, thymus and spleen. Cells isolated from the tissues were cultured with Epstein-Barr virus and the resulting B-cell lines produced Ig in vitro up to 7 weeks, with IgG and IgM anti-TT detected transiently in a culture of cells from the mesenteric membranes.