BACKGROUND The development of a metabolically stable radioiodination reagent for coupling to monoclonal antibodies is a desirable goal. The radioiodination of monoclonal antibodies D612 and 17-1A reactive with human colon cancer with 3-iodophenylisothiocyanate has been investigated. This new ligand, on coupling with monoclonal antibodies, should form a stable thiourea linkage via a reaction of the isothiocyanate moiety with the epsilon-amino group of lysine. METHODS The starting material, 125I- or 131I-labeled 3-iodophenylisothiocyanate, was synthesized in good radiochemical yield with a purity of > 99% via a reaction of electrophilic radioiodine with 3-tri-n-butylstannylphenylisothiocyanate. The coupling of radiolabeled 3-iodophenylisothiocyanate with monoclonal antibodies D612 and 17-1A in different buffers was investigated. Biodistribution of these radioimmunoconjugates in athymic nude mice bearing colon cancer xenografts was studied. RESULTS The results demonstrated that monoclonal antibodies labeled with 3-iodophenylisothiocyanate retained specific binding activity and showed significantly less thyroid uptake than did directly radioiodinated antibodies prepared by the iodogen method. Radioimaging and biodistribution studies demonstrated that uptake of these new radioimmunoconjugates in LS174T colon cancer xenografts was similar to that of directly radioiodinated antibodies, while their uptake in other normal tissues was similar to or lower than that of directly radioiodinated antibodies. CONCLUSIONS These results demonstrate that high specific activity can be achieved and pure 3-iodophenylisothiocyanate can be derived easily from 3-tri-n-butyl-phenylisothiocyanate. Biodistribution and imaging studies revealed that monoclonal antibodies conjugated with 3-iodophenylisothiocyanate are metabolically more stable in vivo in an animal model than directly radioiodinated antibodies, and that these new radioimmunoconjugates are localized selectively in tumors.