Monoclonal antibodies specific to tumor cells may be a useful tool for the diagnosis and treatment of carcinomas. In this study, YPC3 mAb, a specific monoclonal antibody against the human pancreatic carcinoma cell line Capan-2, was labeled directly with 99mTc after being pretreated with 2-mercaptoethanol. The radiolabeling efficiency was found to be high (88%-95%) and the immunoreactivity retained, as assessed by immunohistochemistry and enzyme-linked immunosorbent assay. The biodistribution of 99mTc-YPC3 mAb in nude mice bearing Capan-2 pancreatic carcinoma xenografts was studied and compared with that of labeled mouse immunoglobulin G(99mTc-nmIgG). The xenograft uptake values per gram of tissue 6 h and 24 h after injection of 99mTc-YPC3 mAb were found to be, respectively, 4.72%ID/g and 8.42%ID/g, and the tumor/blood ratios 1.16 and 2.24. At 6 h and 24 h after injection of 99mTc-nmIgG, the uptakes were 2.89%ID/g and 1.80%ID/g, and the tumor/blood ratios 0.84 and 1.02 respectively. External imaging by single-proton emission computed tomography, 24 h after injection of 99mTc-YPC3 mAb, revealed clear scintigraphic visualization of Capan-2 xenografts, whereas injection of 99mTc-nmIgG did not. This study suggests that 99mTc labeling of monoclonal antibody YPC3 might be of practical value in localization diagnosis of human pancreatic carcinoma.