Tissue transplantation immunity with a specific memory component is demonstrated in populations of Montipora. This highly discriminating immunoreactivity derives from extensive allogeneic polymorphism of histocompatibility (H) markers. An H system of immunorecognition is postulated to have originated in multicellular invertebrates probably beginning with coelenterates.

Isografts of Callyspongia diffusa fuse compatibly, but allografts are invariably incompatible. Extensive polymorphism of cell-surface histocompatibility markers is evident. The histocompatibility barriers range from strong to weak depending on the interclonal combination, but early rejection with conspicous cytotoxic sequelae is typical. Reaction times of first-set, second-set, and third-party grafts indicate highly discriminating transplantation immunity with a specific memory component.

No abstract