Lewis rats that were given injections of 106 to 108 (Lewis x Brown Norway) F1 hybrid bone marrow cells produce predominantly, if not exclusively, 19S lymphocyto-toxic antibodies. A number of Lewis rats that received transplants of perfused renal allografts from bone marrow donors at, or near, the peak of IgM response survived for well over 200 days with good renal function and no histological evidence of chronic-rejection. All long-surviving rats had detectable lymphocytotoxic antibodies up to 120 days after allografting; late enhancing antibodies had the restricted specificity possibly identical or similar to anti-I region antisera. All rats bearing prolonged renal allografts were unable to accept donor-specific skin grafts or to respond with specific lymphocytotoxic antibodies following skin grafting. The possible involvement of non-complement-fixing 19S alloantibodies in active enhancement of rat renal allografts is discussed.