Murine monoclonal antibodies represent an attractive type of antitumor therapy because of their potential for exquisite specificity, production in large, pure quantities and mediation of in vivo cytotoxic effects. With maturing monoclonal antibody technology has come the use of these antibodies in clinical studies in patients with malignancy. These trials have established that monoclonal antibodies can be safely administered in large doses, that their pharmacokinetics and tissue penetration can be predicted and that in some instances a therapeutic effect can be produced by their infusion. A number of problems have also been identified by these studies, including antigenic heterogeneity of the tumor, the presence of free serum antigen, the immunogenicity of the xenogeneic antibody, modulation of the surface antigen by the antibody and a finite capacity of human effector mechanisms to mediate cytotoxicity directed by murine antibodies. Other workers are concurrently investigating the use of monoclonal antibodies in the ex vivo elimination of cells from bone marrow, as probes for serum tumor marker antigens and as carriers for radioimaging agents or toxins. Although most of these endeavors are at the earliest stages, promising preliminary results presage an important role for native and altered monoclonal antibodies in the diagnosis and treatment of malignant conditions.