Treatment of BN recipients with soluble WAG/Rij histocompatibility antigens (HCA) did not produce prolongation of (WAG/Rij X BN) F1 heart allografts but resulted in specific sensitization of the recipients in most cases. Three different anti-WAG/Rij sera (ADS) were tested, either alone or complexed in equivalence to 0.5 mg of HCA. Depending on the serum used, ADS alone produced only a moderate prolongation of heart allograft survival or had no effect at all. Antigen-antibody complexes given i.v. on days 0, 2, 4, 6, and 8 induced a prolonged or indefinite cardiac graft survival (greater than 200 days), as well as accelerated graft rejection, depending on the source of the ADS used in immune complex formation. One month after heart transplantation, BN recipients, treated with antigen-antibody complexes, demonstrated normal graft-versus-host reactivity of peripheral blood lymphocytes, but rejected donor-type skin grafts in a slightly delayed fashion.