An isolation method for Tamm-Horsfall protein is described which is based on the observation that a diatomaceous earth filter is able to retain most of the glycoprotein present in urine and that the glycoprotein is easily desorbed from the filter by deionized water. This behaviour depends on the tendency of Tamm-Horsfall glycoprotein at normal urinary concentrations to form a gel in a solution containing mono- and divalent ions. By means of two-step filtration, the glycoprotein was purified to homogeneity. The yield was of about 20 mg/l of urine, and the time required for the isolation was approximately 5-6 h. This procedure should be particularly useful for preparing large amounts of Tamm-Horsfall glycoprotein oligosaccharides in order to investigate their potential use as immunosuppressive agents both in vitro and in vivo.