Gene products of the major histocompatibility complex (MHC) have been shown to elicit lethal graft-versus-host disease (GVHD) in experimental animals. Antibodies specific for MHC cell surface determinants might therefore be expected to overcome histocompatibility barriers and influence survival of marrow graft recipients. GVHD can be consistently induced in dogs by transplanting donor marrow cells into lethally irradiated, unrelated, mismatched recipients. Three anti-Ia monoclonal antibodies were administered to five canine recipients, each at a dose of 0.2 mg/kg body weight per day intravenously for 10 days, beginning on day 0, the day of transplantation. Eight canine recipients were treated with antidog alloantiserum 10 ml/kg body weight per day intravenously on days -2 to day +20, in addition to receiving postgrafting methotrexate. The antiserum was generated by immunizing a matched littermate of the donor with peripheral blood cells of the recipient before transplantation. Survival was no different in the two groups of dogs, compared with historical controls without antibody treatment. A possible explanation for the failure of anti-MHC antibodies to modify acute GVHD in the dog is the inability of antibody to reach critical tissue sites targeted in GVHD.