The Tamm–Horsfall glycoprotein prepared by salt precipitation from urine was found to comprise a heterogeneous collection of aggregates. These could be disaggregated with 8m-urea, following which chromatography on a column of Bio-Gel A.15m yielded a homogeneous glycoprotein of mol.wt. 73000 together with several unidentified impurities. Gel filtration of normal plasma showed the glycoprotein to exist predominantly in a form that is eluted identically with the purified preparation. In one case, material of higher molecular weight was also detected. The purified glycoprotein was used to develop a rapid specific radioimmunoassay for its measurement in human serum or plasma by the use of the Tamm–Horsfall glycoprotein, labelled with 125I by the chloramine-t method as the tracer, an antiserum raised in rabbits, and separation of the bound and free fractions by a second antibody covalently linked to magnetizable particles. Parallelism was demonstrated between the standard preparation and samples. Recovery of added standard to serum varied between 99 and 109%. Total assay time was less than 4h with an intra-assay and inter-assay coefficient of variation of less than 10%. There were no significant differences in the ranges covered with regard to either age or sex, and no circadian rhythm was observed in normal subjects. A physiological range of 70–540ng/ml was established based on serum samples from 95 subjects with normal renal function, as defined by their serum creatinine and urea concentrations. No Tamm–Horsfall glycoprotein was detected in the serum of six anephric patients.

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