A detailed technical protocol is provided for reproducible and aseptical production of stable 186Re-monoclonal antibody conjugates. Labeled Mab E48 IgG and its F(ab')2 fragment which are promising candidates for radioimmunotherapy of squamous cell carcinoma of the head and neck were used for evaluation. S-benzoylmercaptoacetyltriglycine (S-benzoyl-MAG3) was used as a precursor. Rhenium-186-MAG3 was prepared via a unique solid-phase synthesis, after which known strategies for esterification and conjugation to Mab IgG/F(ab')2 were applied. The biodistribution of 186Re-E48 F(ab')2 in tumor-bearing nude mice was found to be comparable to that of analogously labeled 99mTc-E48 F(ab')2 or 131I-E48 F(ab')2, indicating that the intrinsic behavior of the antibody remains preserved when using this labeling technique. Radiolytic decomposition of 186Re-E48 IgG/F(ab')2 solutions of 10 mCi.ml-1 was effectively reduced by the antioxidant ascorbic acid. Upon increase of the Re-MAG3 molar amount, a conjugation of seven to eight Re-MAG3 molecules per Mab molecule was generally the maximum ratio that could chemically be obtained. Such a ratio did not impair the immunoreactivity or alter the in vivo biodistribution characteristics of the immunoconjugate, making this labeling procedure suitable for general clinical application.