p-Aminobenzylethylenediaminetetraacetic acid, synthesized according to the method by Meares, et al. (Anal Biochem 100: 152, 1979), was diazotized and then azo coupled to methyl-p-hydroxybenzimidate to give methyl-(4-hydroxy-3-azobenzylethylenediaminetetraacetic acid)-benzimidate. This compound was conjugated to the lysine residues of an antibody in an attempt to prepare a tumor imaging agent. In these studies, anti human serum albumin was used as a model system. The antibody (2.0 x 10 to the minus 6th power M) was reacted with the imidate at molar ratios of 1:100, 1:500, and 1:1000 for 20 hours in 0.1M borate at pH 8.3 at 23.5 degrees C. The reaction solution was purified by affinity chromatography through a 0.9 x 30 cm combination column containing Sephadex G-50 (24 cm long) and Sepharose 4-B conjugated to HSA (4 cm long). The amount of active or deactivated antibody was detected by a UV monitor. The results showed that conjugation of the chelating agent to lysine residues of the antibody via the amidination reaction at the above reagent ratios did not diminish the antibody-binding activity. Via a mixed anhydride reaction at the same molar ratios, the conjugation of diethylenetriaminepentaacetic acid to the lysine residues of the antibody also did not decrease the antibody-binding activity. In previous studies, we showed that the In-111 labeled diethylenetriaminetetraacetic acid conjugate of human serum albumin (HSA) prepared via the amidination reaction cleared from plasma much faster than I-125 labeled HSA when 1:10 or 1:100 molar ratios of HSA to chelating agent were reacted. This suggests that a specific tumor imaging agent with a high target to blood ratio may be prepared by conjugating a chelating agent to lysine residues of a tumor specific antibody.