SCID mice were injected intraperitoneally with human peripheral blood lymphocytes (PBL) that had been previously stimulated with pokeweed mitogen (PWM) for two days to generate activated B cells. After two weeks, serum and gut washes obtained from SCID mice reconstituted with human PBL (hu-PBL-SCID mice) contained human IgA, IgG, and IgM indicating the successful survival of human lymphoid cell grafts in both mucosal and nonmucosal tissues of the SCID mice. Human IgA plasma cells could be detected by immunofluorescence microscopy in the lamina propria of the small intestine, while IgM plasma cells predominated in the spleen. The results suggested that PWM-activated plasma cell precursors homed to the spleen and the lamina propria of the SCID mouse where they differentiated into plasma cells. In vivo stimulation of hu-PBL-SCID mice with diphtheria and tetanus toxoids (DT/TT) elicited a primary (IgM) immune response pattern rather than a secondary (IgG) response. Antigen-specific antibody-secreting cells were found in the spleen and lamina propria after immunization. The microenvironment of the hu-PBL-SCID mice may select virgin B cells subsets over memory B cell clones.