After orthotopic transplantation of rat livers from Lewis (LEW) donors into Wistar Furth (WF) recipients, a rejection response occurs 2 weeks after transplantation, followed by indefinite survival of the transplant. Spleen cells from WF rats bearing long-term LEW liver grafts responded in MLR and CML assays like spleen cells from normal WF rats after in vitro stimulation with mitomycin C-treated LEW spleen cells, but did not generate CTL after stimulation with paraformaldehyde-treated LEW spleen cells. Transient damage to the long-term grafted livers was induced by inoculation of recipients with homogenized normal LEW liver. CD8+ CTL were detected in the spleen at the time of this liver damage. These findings indicate that CTL precursors are present in WF rats bearing LEW liver transplants. These CTL precursors are capable of differentiating into effector CTL in vitro and in vivo in response to donor antigen. However, once generated, effector CTL in WF rats are eliminated or become unresponsive to the transplanted LEW liver. Homogenized LEW liver from grafted donors did not induce transient liver damage when inoculated into recipient WF rats bearing LEW liver transplants. The demonstration of large amounts of antibody bound to either parenchymal or nonparenchymal cells in the donor graft suggests that reduction (or loss) of antigenicity could also be responsible for the inability of LEW liver to elicit an immune response in WF-recipient rats. Thus, elimination or inactivation of effector CTL and altered antigenicity could both be responsible for maintaining tolerance after allogeneic liver transplantation.