Human mAbs (HumAbs) have therapeutic potential against infectious diseases and cancer. Heretofore, their production has been hampered by ethical constraints preventing the isolation of Ag-specific activated B cells by in vivo immunization. Alternatively, severe combined immune deficient (SCID) mice, transplanted ip with human (Hu)-PBLs, allow the in vivo stimulation of human Ab responses without the usual constraints. Unfortunately, human B cells only represent a minor fraction of the surviving graft, they are scattered all over the animal body, and thus are hard to isolate for subsequent immortalization procedures. To prevent this dispersion and to provide the human B cells with a niche for expansion and maturation, SCID mice were engrafted with Hu-PBL directly into the spleen. Simultaneously endogenous murine NK cell activity was depleted by treatment with an anti-mouse IL-2 receptor β-chain Ab. During engraftment, human B lymphocytes became activated, divided intensely, and differentiated into plasmacytoid cells. In vivo exposure to a recall Ag after cell transfer induced expansion of Ag-specific B cell clones. One week after inoculation, human B cells were abundant in the spleen and could easily be recovered for fusion with a heteromyeloma line. This resulted in the formation of stable hybridoma cell lines that secreted Ag-specific HumAbs. Thus transplantation of human lymphoid cells in the spleens of immune deficient mice represents a model for the study of human T cell-dependent B cell activation and proves to be an excellent tool for the successful production of HumAbs.