The positron emitter zirconium-89 ((89)Zr) has very attractive properties for positron emission tomography (PET) imaging of intact monoclonal antibodies (mAbs) using immuno-PET. This protocol describes the step-by-step procedure for the facile radiolabeling of mAbs or other proteins with (89)Zr using p-isothiocyanatobenzyl-desferrioxamine (Df-Bz-NCS). First, Df-Bz-NCS is coupled to the lysine-NH(2) groups of a mAb at pH 9.0 (pre-modification), followed by purification using gel filtration. Next, the pre-modified mAb is labeled at room temperature by the addition of [(89)Zr]Zr-oxalic acid solution followed by purification using gel filtration. The entire process of pre-modification, radiolabeling and purification steps will take about 2.5 h.