The protocol used for coupling of monoclonal antibodies with mixed anhydride of DTPA for subsequent radiolabeling with indium-111 affects the integrity of the immunoreactivity of the antibody preparations. To analyze the effect of minor methodological variations on coupling characteristics, a two-step addition of DTPA to antimyosin antibody with gentle mixing was compared to a single addition with vigorous stirring. The molar ratios of DTPA to antibody were also varied. The polymer formation was assessed by SDS-PAGE and immunoreactivity was assessed by solid phase radioimmunoassay using human heart myosin as the antigen. The immunoreactivity was significantly decreased in the two-step, gentle-mixing method where polymer formation was evident. The one-step, vigorous-stirring method of DTPA incorporation produced no polymerization and no loss of immunoreactivity.