Monoclonal antibodies may modulate immune and/or biological responses alone, or as carriers of specific agents. Monoclonal antibodies directed against tumours may be indirectly cytotoxic by modulation of antibody-dependent, cell-mediated cytotoxicity or complement-mediated cytotoxicity. Monoclonal antibodies directed against certain tumour cell receptors may alter the biological behaviour of tumour cells such as blocking or downregulation of growth factors essential to tumour cell proliferation. Monoclonal antibodies directed to certain receptors on host immune cells. such as the CD3 receptor on T lymphocytes. may activate those cells and increase their cytotoxicity. Antitumour monoclonal antibodies can serve as carriers of interferons, interleukin-2, tumour necrosis factor and other lymphokines and cytokines to modulate selectively the cytotoxic potential of immune cells in the vicinity of tumour cells. Cytotoxic chemotherapy agents conjugated to antitumour monoclonal antibodies may be processed differently so that they bypass certain mechanisms of drug resistance. The penultimate application of monoclonal antibodies in cancer therapy is to combine various monoclonal antibodies and immunoconjugates for selective combination therapy based on known antigenic tumour cell determinants and the status of the host immune system.