Although iodine-131 is the most widely used radionuclide for radioimmunotherapy, direct radiolabeling methods yield decreased immunoreactivity of the antibody as a function of increased iodine incorporation. We have studied the amino acid sequences of a therapeutic IgG (HuM195), and in particular its complementarity determining regions (CDR), in order to correlate the iodination of tyrosine residues in the antigen binding site with changes in immunoreactivity. The CDR contained an overabundance of tyrosines relative to an expected random distribution of amino acids. In contrast, lysine residues that can be used for ligand attachment were evenly distributed throughout the IgG. HuM195 was first trace labeled with 111In and then labeled with stable 127I at various specific activities. The immunoreactivity of each product was determined using the 111In tracer. The immunoreactivity measured after varying levels of iodination fit a theoretical curve that was generated based on the assumption that a single iodine incorporation anywhere on a tyrosine residue in a CDR destroys the immunoreactivity of the antibody. Similar theoretical curves for antibody fragments (Fab, Fv) suggest an even faster decrease in immunoreactivity with increasing iodination. A review of the sequences of other therapeutic IgG shows that a similar overabundance of tyrosine residues is found in the CDRs. Using enzyme digestion, the distribution of iodine on different parts of the antibody was also studied. The iodinated residues were distributed non uniformly throughout the IgG, with the heavy chain variable region tyrosines having a higher propensity for iodine incorporation than tyrosines in the other regions of the IgG. The common abundance of tyrosine in the CDR of IgG and its correlation with loss of function have important implications for therapeutic use of high specific activity radioiodinated monoclonal antibodies or fragments.