Immune reconstitutions (hu-PBL-SCID mice) resulting from adoptive transfer of human peripheral blood mononuclear cells into 1800 C.B-17 scid-/scid-mice were characterized. Over 90% of reconstitutions were successful as evidenced by human immunoglobulin production. Variability was noted with donor, cell number, and cell type. Human cells (T lymphocytes, few B cells) could be recovered by 5 days after engraftment. High levels of soluble CD8 and interleukin-2 receptors were detected in sera of hu-PBL-SCID mice. Cells recovered from 17 mice proliferated in response to antigens to which the donor had been primed; responses to nonboosted antigen also increased in some animals. After reconstitution, lymphocytes were found in the spleen and lymph nodes without full restoration of normal architecture. The hu-PBL-SCID mouse shows promise as a model system for a variety of immunologic studies. The inherent variation in the system must be minimized for appropriate use of the model.