We describe herein a totally new pathway for the introduction of rhenium in the form of low oxidation state organometallic complexes covalently attached to various proteins. The synthesis of several rhenium conjugates takes advantage of the specificity of N-succinimidyl esters for amino residues. Conjugation experiments were carried out under various conditions, and analysis of the conjugates was performed by FT-IR spectroscopy. Yields were optimized and reached 50%. Furthermore, the conjugate resulting from the coupling of N-succinimidyl 4-[eta 5-cyclopentadienylrhenium tricarbonyl] 4-oxobutanoate to an anti-hTSH monoclonal antibody retained a satisfactory immunoreactivity. Finally, IR detection of conjugates adsorbed onto nitrocellulose membranes was achieved and response was found to be related to the coupling extent of the conjugate.