Because of their high level of specificity monoclonal antibodies can be used as carriers for delivering cytotoxic agents. However they need to have some other features to be usefully used for this purpose. First of all monoclonal antibodies need to be specific for those membrane antigens which are strongly expressed on target cells but absent from normal cells. Secondly it is important to obtain monoclonal antibodies with as a high affinity as possible, and the biodistribution of the molecule recognized by the monoclonal antibody must be considered with the best attention. Chemical treatment of monoclonal antibodies needs to be performed in such a way that neither the antibody nor the toxic agent will result denaturated. Finally, as an alternative to chemically linked immunoconjugates, bispecific antibodies have successfully been used for delivering cytotoxic agents.