Uromodulin is a monomeric glycoprotein excreted in large amounts in urine. Most of the assay methods for quantifying uromodulin rely on the availability of antibodies to this glycoprotein. This study reports a new bioassay for uromodulin without requiring the presence of antibodies. The bioassay is based on the lectin affinity of the glycoprotein and the known interaction between uromodulin and the tumor necrosis factor. The measuring range of the bioassay described in this paper is between 0.13 and 1.25 mug/ml, which makes it a potential method for clinical use.