Monoclonal antibodies, either alone or as immunoconjugates, are now being utilized to treat patients with a wide range of hematologic malignancies. The aim of early studies was to delineate the feasibility of this form of therapy, assess toxicities, and identify potential obstacles. These trials demonstrated that MoABs can be safely administered, and that clinically significant and durable responses occur. Areas of future research include the development of more potent immunoconjugates, the study of MoABs in conjunction with other treatment modalities, and the evaluation of approaches that may eliminate the human anti-mouse antibody response. Methods using MoABs to block growth factor receptors or as vaccines are being considered. At present, MoAB therapy remains investigational and should be restricted to patients with diseases refractory to standard therapies. We are encouraged, however, by the therapeutic results witnessed in early Phase I/II trials.