Monoclonal antibodies of defined specificity can be produced using hybridoma technology. New attempts are being made to explore the potential of antibody therapy for cancer. Preliminary studies show that problems such as antigenic modulation, circulating free antigen, effector cell shortage, and host anti-mouse immunoglobulin response must be overcome. Uses of monoclonal antibodies in cancer treatment include the administration of unconjugated antibodies or of antibodies coupled to drugs, toxins, or radioisotopes, and the use of monoclonal antibodies in vitro to eradicate residual malignant cells.