SUMMARY Four infants with congenital thymic disorders and impaired cellular immunity were treated with 7 separate transplants of fetal thymus and 10 different infusions of hematopoietic tissue (from fetal liver) in an effort to achieve immunological reconstitution. Tissues from immature donors of 7‐22 weeks gestational age were used in three cases with combined immune deficiency disease and in one case of immunodeficiency with normal immunoglobulins (and partial humoral antibody competence). HL‐A typing was carried out in 1 case after several grafts had been attemped. The finding of at least six different antigens suggested lymphoid chimerism from at least two different donors, or a bizarre antigenic composition of the patient's own lymphocytes. Erythrocyte chimerism did not develop in any patient after attempted hematopoietic grafting. Serious graft‐versus‐host reaction (GVHR) was not observed, although two patients had a transient erythrodermia and one developed a Coombs‐positive hemolytic anemia (which could have been either a limited GVHR or an “autoimmune” phenomenon). There was no evidence of immunological reconstitution in any of the four cases. Theories for the lack of significant immunological activity of these grafts are discussed (including the possible role of immunological tolerance of fetal donor tissue, the quantity of graft cells administered, and various host factors).