As an alternative to bone-marrow transplantation, two infants with severe combined immunodeficiency who had no histocompatible donors were given intraperitoneal infusions of fresh liver cells from fetuses of eight and nine to 10 weeks. Transient graft-versus-host disease began at 42 and 52 days, respectively. Both infants had rises in T cells and declines in B cells by three months. No functional immunologic improvement occurred in the first infant, who died of pulmonary disease 10 months later. Clinical and functional immunologic improvement occurred in the other, who is now 19 months after transplantation. Lymphocyte responses to phytohemagglutinin and pokeweed mitogen were noted by three months, to concanavalin A by five months, and to allogeneic cells by eight months. Delayed cutaneous responsiveness to candida developed and IgM became norma. IgA and IgG remained low. Chimerism was demonstrated by a donor marker chromosome in metaphases from recipient lymphocytes. Fetal liver cells therefore reversed the immunodeficiency.