There is an accumulating body of evidence indicating that soluble major histocompatibility complex (sMHC) molecules have donor-specific immunosuppressive properties. One classic, but still unproven, theory is that production of sMHC by liver transplants induces a potent immunosuppressive effect. To mimic this possible effect, we have developed a replication deficient adenovirus (Ad-RQ) to express high levels of donor sMHC class I molecules (sRT1.A(a)) in the liver. Ad-RQ produced sRT1.A(a) was measured by enzyme-linked immunosorbent assay (ELISA) after in vitro infection of Lewis (RT1(l)) hepatocytes, and in vivo following intravenous virus injection into Lewis rats. Results indicated high sRT1.A(a) expression in Lewis hepatocyte cultures and, in vivo, high expression was also demonstrated and maintained for at least 1 week. A strong immunosuppressive potential of sMHC in vivo was revealed by prolongation of cardiac (ACI, RT1(a)) heart allograft survival in high-responder Lewis rat recipients treated with Ad-RQ alone. Furthermore, limiting dilution cytotoxic T-lymphocyte precursor (CTLp) analysis of lymphocytes from Ad-RQ-treated Lewis recipients receiving an ACI heart transplant indicated a marked decrease in antidonor CTLp frequency. In conclusion, our results demonstrate that viral vectors can be used effectively to express high levels of sMHC molecules, and their immunosuppressive effect, without concurrent immunosuppression, is sufficiently potent to prolong heart transplant survival.