Human, chicken, and guinea pig corneas organ cultured (O.C.) from 1 to 4 weeks were transplanted intralamellarly into rabbits. Chicken and guinea pig corneal xenografts O.C. 3 to 4 weeks had statistically significant delayed rejection times compared to fresh controls. In addition, 22% of chicken xenografts O. C. 4 weeks did not reject. Histologically, O.C. xenografts with delayed rejection or nonrejection were hypocellular. Chicken xenografts O.C. 4 weeks in which recipient (autochthonous) rabbit serum replaced the calf serum routinely used in the media also had a significant delay in rejection time when compared to fresh controls or chicken xenografts stored in pooled (allogeneic) rabbit serum. The rejection time of human corneal xenografts O.C. for periods up to 4 weeks was not delayed. These data suggest that in this model, prolonged survival of xenografts after O.C. is species specific and represents a form of immunological modification, possibly reduced antigenicity secondary to donor hypocellularity.