In addition to encoding the well-known class I (H-2), II (Ia), and III (complement components C2, C4, and factor B) antigens, the murine MHC controls the expression of a large, intracellular protein complex of unknown function. This complex is composed of a large number of noncovalently linked low molecular weight polypeptide subunits (hence the name, LMP) which are biochemically, serologically, and genetically distinct from class I, II, and III antigens. Only two of these subunits display electrophoretic polymorphism within the standard inbred mouse strains, and both of these polymorphisms map within the H-2 complex, between the H-2K and I-A subregions. The remainder of the LMP complex subunits have not been mapped, and may be encoded elsewhere in the genome. A biochemically similar complex has been detected in human cells, although linkage to HLA remains to be established. In this article we will review the biochemistry, serology, and genetics of the LMP antigens, and will speculate on their biological function.