A simple method for conjugation of monoclonal antibodies (mAbs) with the chelating agent 1, 4, 8, 11-tetraazacyclotetradecane-l, 4, 8, 11-tetraacetic acid (TETA), has been developed using commercially available reagents. This method involved activation of a single carboxyl group of TETA with N-hydroxysulfo-succinimide and 1-ethyl-3-[3- (dimethylamino)propyl]carbodiimide. The resulting activated ester of TETA was reacted with the anti-colorectal carcinoma mAb 1A3 at molar ratios ranging from 10:1 to 100:1 to give immunoconjugates modified with an average of 0.4 to 2.0 functional chelators per antibody molecule. The TETA-1A3 conjugate was labeled with 64Cu at specific activities as high as 15.4 μCi/μg, and the radiolabeled mAb exhibited high in vitro serum stability and minimal loss of immunoreactivity. The biodistribution of 64Cu-labeled TETA-1A3 in hamsters bearing GW39 human colon carcinoma xenografts was compared to that of 64Cu-BAT-2IT-1A3 (BAT = 6-(p-bromoacetamidobenzyl)-l, 4, 8, 11- tetraazacyclotetradecane-1, 4, 8, 11-tetraacetic acid; 2IT = 2-iminothiolane). Both conjugates showed high tumor up-take (6.60-9.05% injected dose/gram) from 24 to 48 h post-injection and generally similar blood clearance and non-target organ uptakes. Human absorbed dose estimates derived from the hamster biodistribution data showed the critical organs for both conjugates to be the large intestine and the red marrow. Our results suggest that the in vitro and in vivo performance characteristics of 64Cu-TETA-1A3 compare favorably with those of 64Cu-BAT-2IT-1A3 and that further evaluation of the diagnostic and therapeutic efficacy of 64Cu-TETA-1A3 is warranted.