A simple spectrophotometric method was developed to quantitate micromolar concentrations of a bifunctional DTPA ligand in DTPA monoclonal antibody (mAb) conjugates. Titration of a brightly colored 1:2 yttrium (III) complex of arsenazo III with the ligand 1B4M-DTPA obeyed Beer's law over the concentration range 0-2.0 microM 1B4M-DTPA at 652 nm. From a calibration plot of absorbance versus 1B4M molarity, concentrations of 1B4M-DTPA conjugated to mAb were determined. Mole ratios of 1B4M-DTPA to mAb agreed satisfactorily with the ratios obtained by a radioanalytical technique using carbon-14-labeled 1B4M-DTPA and a binding assay using 111In. The spectrophotometric method was applied successfully to the preparation of 1B4M-DTPA mAb anti-TAC, a mAb conjugate used in clinical trials of 90Y radioimmunotherapy.