In a previous study on homologues of the human Rh-Hr types in apes, weak agglutination of orangutan red cells by human anti-Rho and anti-hr’ sera was observed. Lack of adequate amounts of orangutan red cells needed for absorption experiments did not allow to ascertain whether the weak reactions were due to variants of the Rho and hr’ factors on orangutan red cells or to anti-species (species-specific) agglutinins against orangutan red cells in the human typing sera. The present experiments indicate that homologues of the human blood factors Rh0 , hr’ as well as rh’, rh” and hr” are lacking in orangutan red cells which are, therefore, comparable to the human Rh null red cells. In the search for homologues of chimpanzee C-E-F blood groups in other ape species, tests on 15 gorillas disclosed the presence in gorillas of polymorphism for Cc and Fc, while cc, antithetical to Cc in chimpanzees, as well Ec were both found to be absent from gorilla red cells.