The performance characteristics of Milenia TSH (Diagnostic Products Corporation, Los Angeles, CA), a kinetic enzyme immunoassay for the quantification of thyroid stimulating hormone (TSH) were investigated. Evaluation included assessment of precision, specificity, sensitivity, high dose hook effect, and comparison of results to those of other technologies. Precision was found to be acceptable. The assay was determined to be highly specific for TSH with low crossreactivity to structurally related glycoprotein hormones such as human chorionic gonadotropin, luteinizing hormone, and follicle stimulating hormone. Sensitivity was sufficient to discriminate between hyperthyroid and euthyroid samples. The high dose hook effect, characteristic of immunometric assays, was found at a concentration sufficiently high to be of little or no concern. Results obtained on the method were compared to those from three other commercially available methods for TSH from Diagnostic Products Corporation, London Diagnostics, and Abbott Laboratories. Values generated on the Milenia TSH assay were in good agreement with those of other methods.