A radioimmunoconjugate consisting of a humanized monoclonal antibody directed against the extracellular domain of prostate-specific membrane antigen linked to a beta-emitting radioisotope (lutetium-177). This radioimmunoconjugate binds to tumor cells that express the extracellular domain of prostate-specific membrane antigen, delivering beta particle radiation selectively to tumor cells expressing this antigen and so limiting the exposure of normal tissues to ionizing radiation.