Abstract: A two-site "Sandwich" immunoradiometric assay for TSH with use of magnetizable particles is developed. The assay involves monoclonal antibody and polyclonal antibody. The monoclonal antibody is Iabeled with ~(125)I. The polyclonal antibody is conjugated covalently to magnetizable particles. Sensitivity is 0.04 μIU/ml, Intraassay coefficient of variation is 4.6%—10.0%. Interassay coefficient of variation is 7.1%—10.2%. Average recovery is more than 95%. The effects of assay procedure, incubation time and protein concentration on results are discussed.