Evidence for quasi-silent germline genes coding for phylogenetically ancient determinants of the rabbit a locus allotypes.

Anti-a2 sera raised in a3 rabbits are shown to detect determinants on a2 molecules which are different from those detected by anti-a2 sera raised in a1 animals. The former determinants are occasionally observed at a low level in rabbits of the a1 allotype and at a high level in sera of Leporidae of different genera. The two types of anti-a2 sera are shown to compete for the same sterical region of the a2 molecules. All homogeneous a2 molecules which have been tested show both types of determinants.