Monoclonal antibodies can be directly labelled with 188Re using a simple one-step radiolabelling kit. Using B72.3 as a model antibody, the formulation was optimized and kits were made and tested and compared to data previously reported for the same antibody labelled with other radioisotopes. Labelling with Re-188 was carried out with the eluate of a W-188/Re-188 generator from Oak Ridge National Laboratory. Fresh generator eluate was added to the pre-reduced lyophilized antibody and the mixture allowed to incubate overnight at room temperature. The radiochemical purity, immunoreactive fraction, and biodistribution in normal and LS174T tumor bearing nude mice was determined. The radiochemical purity was 88 +/- 7%, the immunoreactive fraction was 68.46 +/- 3.8%. The immunoreactive fraction was higher than any previously reported for this antibody when labelled with other radioisotopes. At 48 h, 7.9 +/- 2.4% of the injected dose per gram was found in the tumor. The biodistribution and tumor uptake of Re-188 labelled B72.3 was similar to that previously reported for Re-186 and In-111 labelled B72.3.