H-2.28, an alloantigenic marker allelic to H-2.1, is expressed on all three known types of H-2 molecules.

Each allele at the K or D region of the H-2 complex produces two kinds of "allelic" or mutually exclusive antigenic characteristics: its unique private specificity and a public specificity(ies) of either the H-2.28 or H-2.1 family. The private specificities of the K and D regions are expressed on H-2K and H-2D molecules, respectively. The D region produces another molecule, H-2L, which lacks the H-2K and H-2D private specificity but exhibits the H-2.28 or H-2.1 specificity. We analyzed the expression of the H-2.28 determinants on H-2K, H-2D, and H-2L molecules. When an antiserum against H-2.28 is used to sensitize cells where it can react with only H-2K molecules or H-2D molecules, by subsequent elution antibodies against H-2.28 are recovered that can also react with H-2L molecules. Hence, determinants reactive with antibodies against H-2.28 are present on H-2L as well as on H-2K and H-2D molecules. The expression of the H-2.28/H-2.1 polymorphism on all three known types of H-2 molecules, without some obvious relation to the private specificities, suggests that the antigenic determinants of these two kinds of allelic systems (private in contrast to H-2.28/H-2.1) may be controlled by separate genes, even when they are expressed on the same molecule.