Rat hearts were transplanted across major histocompatibility loci in two donor‐recipient combinations. A pronounced and specific suppression of rejection was obtained when recipients were treated with one injection of bone marrow cells 7 days prior to allografting. Histological signs of immune reactions in the grafts were found and some rats produced circulating alloantibodies. Skin grafts were rejected in the one combination but were accepted in the other, when applied to recipients with well tolerated hearts of the same genotype. Normal response of recipient spleen cells in GVHR was obtained, and parabiotic union between recipient with functioning grafts and normal unmodified rats did not effect function of the transplants. The suppressive effect of marrow pretreatment could not be transferred with serum taken from rats at the time of transplantation. The possible mechanisms are discusse