T cell-depleted haploidentical (parental) bone marrow stem cell transplants are given to most infants with the syndrome of severe combined immunodeficiency (SCID) because they have no available HLA-identical sibling potential donors. Since they usually do not undergo cytoreduction prior to transplantation, these children later demonstrate mixed hematopoietic chimerism. Most often, T cells (but usually not B lymphocytes, macrophages, or other hematopoietic cells) can be shown to be of donor type. The origin of natural killer (NK) cells in such chimeras has not been reported. Two lymphocyte lines derived from the CD16+ fraction of an adenosine deaminase (ADA)-deficient male SCID's blood mononuclear cells (MNC) 13 months following maternal marrow stem cell transplantation demonstrated typical phenotypic and functional characteristics of NK cells after expansion. Karyotyping showed both lines to be XX. Thus, NK cell engraftment can occur in SCID infants who have not been conditioned, even when significant NK cell function is present before transplantation.