A method of selecting a test molecule that binds to HLA-DP4 by (i) incubating HLA-DP4 with the test molecule and a labeled peptide of formula (I) Z.sub.1X.sub.1X.sub.2X.sub.3X.sub.4X.sub.5X.sub.6X.sub.7X.sub.8X.sub.9Z.s- ub.2 thereby forming respective complexes, wherein Z.sub.1 and Z.sub.2, which may be identical or different, are each either zero or from 1 to 100 amino acids; X.sub.1 is an aromatic or hydrophobic amino acid, or S; X.sub.6 is an aromatic or hydrophobic amino acid, or C; X.sub.9 is an aromatic or hydrophobic amino acid, or C, D, Q, S, T, or E; and X.sub.2, X.sub.3, X.sub.4, X.sub.5, X.sub.7and X.sub.8 are each an amino acid; (ii) separating the respective complexes formed; (iii) detecting the HLA-DP4/labeled peptide complexes; and (iv) selecting the test molecule that exhibits a binding activity of IC.sub.50<1000 nM, which corresponds to the concentration of the test molecule that inhibits 50% of the competitive HLA-DP4 binding of the labeled peptide.