Total lymphoid irradiation (TLI) is a method which delivers irradiation daily in fractionated doses (200 rads) to lymphoid organs while shielding bones, lungs, and the majority of the gastrointestinal tract. TLI is lymphocytopenic in mice, rats, dogs, and humans, and both T cells and B cells are eliminated from the circulation. During the recovery phase in mice, B cells appear before Thy 1.2-bearing T cells, and TL-positive T cells are abundant in the spleen and lymph nodes. These T cells exhibit nonspecific suppressive activity on antibody production and cell-mediated immune responses. TLI permits establishment of specific and long-lasting tolerance to alloantigens. Permanent acceptance of allogeneic bone marrow cells without graft-versus-host disease was achieved in rats and dogs across major histocompatibility barriers. Recipients were tolerant to allografts of skin, hearts, and kidney from animals syngeneic to marrow donors or to organs from the marrow donor. This approach may be suitable for pancreas transplantation in diabetes.