Primed Lymphocyte Typing (PLT) with local (CDP) and the 9th International Histocompatibility Workshop reagents (GNN) revealed "cross-reactions" between HLA-DPw6 and the GNN2B but not other DPw2 PLT-cells (GNN2A, CDP2A, CDP2B). We raised and bulk-expanded a well discriminating PLT-reagent (the JET-reagent) from an HLA-DR identical, DP, GNN2A, CDP2A, CDP2B compatible and GNN2B incompatible responder/stimulator combination. The JET-reagent defined a "new" determinant, JET, which was present in 11% of Danes. The JET determinant was associated with DPw2, w6, and DP-blank: 66% of DPw6 and DP-blank and 20% of DPw2-positive individuals were JET-positive. All JET-positive individuals belonged to this group, and six of these had two DP-antigens. JET segregated with DP-blank in three and with DPw6 in two informative families. In an HLA-DR/DPw6 recombinant family, JET segregated with DPw6 in the recombinant haplotype. The JET PLT-responses against all of six different JET-positive stimulators including the specific stimulator were strongly inhibited by the monoclonal antibody (MoAb) Tü39, which preferentially reacts with DP-molecules, but not by other HLA-specific MoAbs. The results indicate that JET is either a "new" supertypic DP-related specificity or a determinant, which shares epitopes with DP, of a "new" locus and in linkage disequilibrium with DP.