The optimum labelling condition was established for using a new bifunctional chelating agent （DOTA-Peptide） in 90Y-labelled monoclonal antibody Lym-1 by the pre-labelling method

in which the 90Y was first labelled to the bifunctional chelating agent and then conjugated to the monoclonal antibody. The results showed that the radiolabelling procedure was reasonable and the radioactivity yield in products was satisfactory. The radiochendcal purity of 90Y-labelled Lym-1 was determined to be over 95% by gel filtration HPLC and silica gel TLC. The immunoreactivity of the final product was found to be greater than 100% relative to 125I-Lym-1 （as a standard） by in vitro cell binding assay.