Female nu/+ or BALB/c mice were immunized with human peripheral blood lymphocytes (PBL) before and during pregnancy. Pups born to these mothers were inoculated with human PBL or human fetal bone marrow and thymus cells. Tolerization of the pups to human PBL was observed without graft-versus-host reaction. Presence of human immunoglobulins was observed in the pups for 3-4 weeks. Human T cells also could be detected for a period of 3-4 months in these mice.