99Tcm is the optimum radionuclide for imaging in nuclear medicine due to its superior physical properties (E gamma of 140 keV and t1/2 of 6 h). Several techniques have recently been developed for labelling monoclonal antibodies with 99Tcm for immunoscintigraphy of human malignancies. These techniques primarily consist of either direct labelling of endogenous sulphydryl groups on the immunoglobulin with 99Tcm or indirect labelling through conjugation of a preformed 99Tcm-chelate. Direct methods offer the best promise for a one-step labelling kit but the 99Tcm-antibody may be unstable in vivo. This instability has been advantageous, however, in reducing blood background radioactivity and achieving sufficiently high tumour/blood ratios for clinical imaging. Over 1200 patients have been studied with 99Tcm-labelled monoclonal antibodies in the past decade. The majority of studies have been carried out in melanoma or colon cancer but other malignancies have also been investigated. The sensitivity has been variable and depended both on the size of the lesion and its location. Single photon emission computed tomographic imaging was helpful in some instances. Further study of labelling techniques and their effect on the pharmacokinetics of 99Tcm-labelled monoclonal antibodies as well as additional clinical evaluation of these agents is indicated.