The haplotype binding pattern of a new mAb RIB5/1 against RT1 class II antigens on different tissues of inbred rat strains was investigated using the indirect POD-technique and a cytofluorometric analysis. RIB5/1 binds to a polymorphic determinant of the RT1 class II glycoprotein. Unlike MRC OX-3 (and OX-6), RIB5/1 reacts in optimal dilution with the RT1 haplotypes dvl, a, av, g, 1 but not with u. Therefore, the mAb pair MRC OX-3 (OX-6) and RIB5/1 is applicable for specific immunohistological purposes in chimaeric models with these RT1 haplotypes.