A procedure for modifying a donor immunoglobulin for use in a target species, a procedure comprising the steps of: - identifying a donor immunoglobulin of a species other than the target species, in which the donor immunoglobulin has binding specificity to a target epitope present in the target species, - determine an amino acid sequence of conserved framework regions of the variable domains of the heavy and light chains of the donor immunoglobulin, - compare each amino acid residue of the amino acid sequence of the conserved framework regions of the domains heavy and light chain variables of the donor immunoglobulin with an amino acid residue present in a corresponding position in an amino acid sequence of conserved framework regions of a plurality of immunoglobulins obtained from the target species to identify one or more amino acid residues within the amino acid sequence of the conserved framework regions of the variable domains of the heavy and light chains of the donor immunoglobulin that is not present at the corresponding position in the amino acid sequence of the conserved framework regions of at least one of the plurality of immunoglobulins obtained from the species target, and - replace the one or more identified amino acid residues present in the amino acid sequence of the conserved framework regions of the variable domains of the heavy and light chains of the donor immunoglobulin, but not present in the corresponding position in the sequence of amino acids of the conserved framework regions of at least one of the plurality of immunoglobulins obtained from the target species, with an amino acid residue that is present in the corresponding position in the amino acid sequence of the conserved framework regions of at least one of the plurality of immunoglobulins obtained from the species Diana； - in which the modified donor immunoglobulin does not contain any amino acid in any position within the conserved framework regions that might be foreign in that posi
