Marrow grafts were carried out between 16 canine sibling donor-recipient pairs. The pairs were matched by serological histocompatibility testing and were nonreactive in a one-way mixed leukocyte culture. Recipients were prepared for transplantation by 1500-1580 R of total body irradiation. Donor marrow was infused within 4 hr of irradiation. Recipients were not given immunosuppressive drug therapy after grafting. All 16 recipients showed evidence of prompt and sustained allogeneic marrow engraftment. Six died between 30 and 128 days after grafting with graft-versus-host disease, and three died between days 72 and 230 with pneumonia but no evidence of graft-versus-host disease with the exception of lymphoid atrophy. Seven recipients survived without graft-versus-host disease and are in excellent health between 200 and 684 after grafting.

In summary, fatal graft-versus-host disease was observed in a number of canine recipients despite matching with their sibling donor by serological histocompatibility testing and by mixed leukocyte culture in a manner similar to that employed to define human HL-A matched sibling pairs. The graft-versus-host disease in these matched siblings developed more slowly than that observed in mismatched dogs, but the ultimate death of approximately half of the matched recipients emphasizes the need for posttransplantation immunosuppression even in this “compatible” situation.