Introduction: Monoclonal antibodies labeled mainly with 99mTc are being widely used as imaging agents in nuclear medicine. Recently PR81 was introduced as a new murine anti-MUC1 MAb against human breast carcinoma. This antibody reacts with the tandem repeat of 20-mer peptide of protein backbone, and is of IgG1 class, with an affinity of 2.19×10-8 M-1. Due to high specific reactivity, this antibody was shown to have high diagnostic potential in nuclear medicine. As the first step towards the use of an antibody for imaging purposes it has to be labeled with a radioisotope. In this study we have investigated a method for optimum labeling of this MAb with 99mTc. Materials and Methods: The Ab reduction was performed with 2-mercaptoethanol (2-ME) at a molar ratio of 2000:1 (2-ME:MAb) and reduced Ab was labeled with 99mTc via stannous tartrate as a transchelator. The labeling efficiency was determined by ITLC. The integrity of reduced MAb was checked by means of SDS-PAGE and gel filtration chromatography (FPLC) on Superose 12 HR 10/30 (purity>99%). The amount of radiocolloids were measured by cellulose nitrate electrophoresis. In vitro stability of labeled product was checked in time intervals over 24 hrs by ITLC. Radioimmunoassay was used to test immunoreactivity of the labeled MAb. Biodistribution of the radiolabeled MAb was studied in normal mice at 4 and 24 hrs post-injection. Results: The labeling efficiency was %96±2.8 with high invitro stability (%82±2.7 in 24 hrs) and less than %2 radiocolloids were found. There was no Ab fragmentation due to reduction procedure. Both labeled and unlabeled MAbs were able to compete for binding to the MUC1. Biodistribution studies in normal mice showed that there was no significant accumulation in any organ. Conclusion: Because of no significant loss of immunoreactivity of MAb due to labeling procedure, high in vitro stability and no accumulation in vital organs, 99m-Tc-PR81 may be a promising candidate for radioimmunoscintigraphy studies of breast cancer.