The present invention provides a cyclic polypeptide fragment of CD20 comprising (f) a contiguous amino acid sequence consisting of amino acid residues SEKNS (SEQ ID NO:67}； (if) a first cysteine residue which is present at a region N-terminal to the contiguous amino acid sequence and (iif) a second cysteine residue which is present at a region C-terminal to the contiguous amino acid sequence, wherein the cyclic polypeptide is oxidised by the presence of a disulphide bond formed between the first and second cysteine residues. Also described is the use of the cyclic peptide fragment to generate antibodies which bind specifically to CD20. Antibodies which bind specifically to the cyclic peptide fragment for use in treatment of B-cell mediated conditions in felines and canines are also described.
