The present invention relates to disulphide bond stabilized recombinant MHC class II molecules. In particular, the present invention provides a recombinant MHC class II molecule, which comprises: (i) all or part of the extracellular portion of an MHC class II α chain; (ii) all or part of the extracellular portion of an MHC class II β chain; wherein (i) and (ii) provide a functional peptide binding domain and wherein (i) and (ii) are linked by a disulphide bond between cysteine residues located in the α2 domain of said α chain and the β2 domain of said β chain, wherein said cysteine residues are not present in native MHC class II α2 and β2 domains. Methods of producing these molecules in prokaryotic systems and various uses of these molecules form further aspects.