Fetal thymus transplantation was performed in three patients with thymic hypoplasia with abnormal immunoglobulin synthesis, one patient with ataxia telangiectasia, and one patient with immunodeficiency with eczema and thrombocytopenia. All patients received transfer factor before transplantation of a fetal thymus i.p. Reconstitution of cell-mediated immunity occurred in three of five patients. Two of the three patients with reconstitution of cell-mediated immunity also had evidence of improved antibody-mediated immunity. Reconstitution of cell-mediated immunity was characterized as occurring rapidly and being of varying duration, and was unassociated with HL-A chimerism. Successful reconstitution of immunity in these patients may have been related to several factors, including the use of fetal thymus less than 6 hr after abortion, i.p. transplantation, and a synergistic effect of transfer factor.