Protein, or parts of proteins, may be rendered non-immunogenic, or less immunogenic, to a given species by identifying in their amino acid sequences one or more potential epitopes for T-cells of the given species and modifying the amino acid sequence to eliminate at least one of the T-cell epitopes. This eliminates or reduces the immunogenicity of the protein when exposed to the immune system of the given species. Monoclonal antibodies and other immunoglobulin-like molecules can particularly benefit from being de-immunised in this way: for example, mouse-derived immunoglobulins can be de-immunised for human therapeutic use.