The monoclonal antibody 14-4-4S reacts with the I-Ek class II alloantigen on cells from mice of the H-2d, H-2p, and H-2r haplotypes.1 Cells from mice of the H-2b, H-2f, H-2q, and H-2s haplotypes do not express I-E antigen.2 The antibody has been reported to cross-react with the rat MHC class II alloantigen RT1D.3