Bifunctional chelators for labeling antibodies with 99mTc based on the N3S core of (mercaptoacetyl)-triglycine having ester or amide linking moieties were synthesized and site-specifically attached to the sulfhydryl groups of the Fab' fragment of antimyosin. Protein labeling was quantitative after 15 min; postlabeling purification was not necessary. The radiolabeled conjugates exhibited no loss of immunoreactivity. Under basic conditions, the ester-linked conjugate lost 95% of the radiolabel in the form of the 99mTc complex of (mercaptoacetyl)triglycine as determined by RP-HPLC, while the radioactivity in the amide-linked conjugate remained completely bound to the protein. In a mouse biodistribution study, the ester-linked conjugate showed a 2-fold enhancement in clearance from the kidney when compared to the amide-linked product.