A study was made of the covalent attachment of the bifunctional 2-(4-isothiocyanatobenzyl)diethylenetriaminetetraacetic acid family of chelate ligands to proteins for the purpose of labeling monoclonal antibodies with radiometals. The parameters and the chemical variables examined included pH, reaction period, temperature, and ligand and protein concentrations. It is shown that these variables, with the exception of protein concentration, have significant effects on the rate of protein conjugation. Conjugation of three monoclonal antibodies and human IgG under identical conditions showed only 17% variation. Finally, the effect of the concentration of conjugated IgG on radiolabeling yield was studied.