There is a need to improve on existing targeting technologies in order to develop effective cancer therapy. We have investigated this for colorectal cancer using antibodies directed against carcinoembryonic antigen (CEA). Chemical and molecular protein engineering has been used to produce antibody molecules which differ in molecular weight, affinity, valency and specificity. These have been characterised and tested in animal tumour models and clinical trials to test the parameters important for optimising tumour penetration, increasing residence time in viable areas of the tumour, accelerating clearance from normal tissues and improving therapeutic efficacy.