A variety of materials have been suggested as carriers for the delivery of drugs to specific sites of action. Drugs may be covalently bound to carriers or physically trapped within particulate carriers. Likely mechanisms of action of targeting agents are described. The results of in vitro and in vivo experiments are reviewed and the prospective clinical uses of each type of carrier are discussed. In particular, monoclonal antibodies are promising agents for the targeting of cytotoxic agents to malignant cells. Radiolabelled monoclonal antibodies are likely to develop as agents for the radio-imaging of tumours which will prove useful in the diagnosis and treatment of cancer.