Monoclonal antibodies are playing an increasing role in many experimental therapies. A major limitation of their use is that they are recognised by the patient as being of foreign origin and an antiglobulin response is provoked. Recombinant DNA technology offers the ability to convert these rodent antibodies into a more human form. Different degrees of humanisation can be achieved ranging from chimeric antibodies with a combination of human constant regions with rodent variable regions to fully reshaped antibodies where the variable regions are also humanised. Encouraging preliminary results have been obtained and it seems likely that these will dictate the use of humanisation as a required procedure in the therapeutic use of monoclonal antibodies.