AbstractAnti‐a 2 sera raised in a3 rabbits are shown to detect determinants on a2 molecules which are different from those detected by anti‐a 2 sera raised in al animals.The former determinants are occasionally observed at a low level in rabbits of the al allotype and at a high level in sera ofLeporidaeof different genera. The two types of anti‐a 2 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 determi