Abstract: An IRMA for determination of CA19-9 in human serum based on two monoclonal antibodies are developped, one of which is coated on polystyrene tubes as solid phase and another labelled with ¹²⁵I as the tracer. Two-step assay procedure is employed and mouse IgG is used to block heterophilic antibody interference. The intra- and inter- assay of the IRMA are 6.4%~9.5% and 6.0%~12.6% respectively, the detection limit of the assay is 2.0 U/mL, and recovery is 88.1%~106.1%. The results of dilution test demonstrate good correlation between dilution times and values (r=0.990).