The ability of a radiolabelled monoclonal antibody, LICR-LON-M8 (M8), to locate metastatic breast carcinomas has been investigated. The scans generated by M8, either when labelled with radioiodine, or when conjugated with diethylenetriamine-pentaacetic acid (DTPA) and labelled with radioactive indium (111In), have been compared with X-rays and 99mTc-methyl diphosphonate (MDP) bone scans. All 10 patients with skeletal metastases had positive 111In-DTPA-M8 scans and the overall correlation with X-rays and MDP scans was good but varied with the region studied. By contrast, radioiodinated M8 did not detect metastases at any site. The discrepancies between 111In-DTPA-M8 images and conventional techniques may be related to the different stages in the evolution and development of the lesion at which the various techniques detect bone metastases.