To further expand the range of analytes that can be detected by using ICP-MS coupled with bioanalytical methods, we have employed a new separation system based on the highly active surface streptavidin and biotinylated monoclonal antibody (McAb) in a competitive immunoassay followed by ICP-MS detection. Specifically, we have demonstrated its application for the determination of total thyroxine (T₄) in human serum using Eu³⁺ as a label. In this method, streptavidin immobilized to the pre-coated bovine serum albumin (BSA)-biotin on the microwells showed a significantly higher binding capacity for the biotinylated anti-T₄ monoclonal antibody. Total T₄ was quantified by measuring the Eu³⁺ intensity with ICP-MS after using 1% HNO₃ to extract Eu³⁺ from the bound fraction of the immunocomplex. The detection limit was 7.4 ng mL⁻¹ with a sample volume of 25 µL. Total T₄ values obtained by this procedure agreed well with those obtained by a chemiluminescent immunoassay method. Both inter- and intra-assay precisions were below 10%. The results indicate that a general competitive ICP-MS-based immunoassay scheme may be possible.