OBJECTIVE To determine the optimal conditions for enhancing corneal allograft survival by oral immunization with donor-specific alloantigens. METHODS CB6F1 mice were orally immunized with various doses of C3H/Hej corneal epithelial and endothelial cells before receiving orthotopic C3H/Hej corneal allografts. Paraformaldehyde-fixed corneal cells were compared with viable corneal cells for their capacity to promote corneal allograft survival. The mucosal adjuvant, cholera toxin B (CTB), was examined for its capacity to enhance corneal graft survival when given separately or conjugated to corneal cells used for oral immunization. Oral immunization was also evaluated for its capacity to prevent immunologic rejection in three high-risk settings: preimmunized hosts, hosts with prevascularized graft beds, and grafts that contain donor-specific Langerhans' cells. RESULTS Optimal graft survival occurred when 2 x 10(6) corneal cells were administered orally 10 days before orthotopic corneal transplantation. Paraformaldehyde-fixed corneal cells were as effective as viable cells in preventing corneal graft rejection. Cholera toxin B enhanced the efficacy of oral immunization when conjugated with the orally administered corneal cells but was ineffectual when administered separately. Oral immunization with donor corneal cells enhanced corneal graft survival in all three high-risk settings. CONCLUSIONS Oral immunization with donor cells is an effective strategy for enhancing corneal graft survival and preventing graft rejection in high-risk settings. Graft enhancement is optimized when the orally administered cells are conjugated with CTB and administered before corneal transplantation. Because fixed cells retain their capacity to enhance corneal graft survival, it may be possible to store donor cells for long-term use in high-risk hosts.