Disclosed is a method of preparing an antibody or an antigen binding fragment thereof that specifically binds to a cyclic polypeptide fragment of CD20 from a target species, the method comprising the steps of: - assessing the sequence of the framework regions of a donor antibody from a species other than the target species, wherein the donor antibody has binding specificity for CD20 from the target species - comparing the sequence of the framework regions of the donor antibody with the sequence of the framework regions of an antibody or a pool of antibodies derived from the target species and - modifying the framework regions of the donor antibody to substitute amino acid residues that are foreign at a corresponding position in an antibody or a pool of antibodies from the target species with amino acid residues present at the corresponding position in the target species wherein the modified framework regions do not contain any amino acids in any position that would be foreign at that position in the target species. Also disclosed is a caninised/felinised antibody or an antigen binding fragment thereof which binds specifically to a cyclic polypeptide fragment of canine CD20, wherein the antibody comprises complementarity determining regions of a heavy and a light chain from a donor antibody from a species other than a canine, wherein the donor antibody has binding specificity for the cyclic polypeptide fragment of canine CD20, wherein the framework regions of the heavy and light chains from the donor antibody are modified to substitute amino acid residues that are foreign at a corresponding position in canine antibodies with amino acid residues present at the corresponding position in canine antibodies, and wherein the modified framework regions do not contain any amino acids in any position that would be foreign at that position in canine antibodies.
