Monoclonal antibodies directed at tumor-associated antigens have been used clinically since 1981. In most of the completed clinical trials, the McAbs were of murine origin. Twenty-six of 184 patients (14%) receiving McAbs demonstrated a major clinical response, including 3 complete responders. Toxicities are primarily related to immune responses. The immune response triggering the toxic manifestations may be mediated by interactions between the administered McAb and the target tumor-associated antigens, or between the McAb and host antibodies made in response to its administration. Analysis of the mechanism of action, pharmacokinetics and toxicity of McAb administration should lead to improved design of clinical protocols, and provide a basis for the use of human McAbs and McAbs conjugated to radioisotopes, toxins or chemotherapeutic agents.