1. A quantitative radioimmunoassay was developed for the measurement of nanogram amounts of Tamm—Horsfall (TH) glycoprotein in the presence of serum proteins at concentrations above 30 mg/ml.

2. Specific anti-(TH-glycoprotein) antibodies were labelled with 125I and these were usable for a period of 8 weeks.

3. Agarose beads (Sepharose 4B), to which TH-glycoprotein had been coupled via cyanogen bromide activation of the Sepharose, were used as the solid phase in the assay. This proved stable upon storage at 4°C for periods in excess of 4 months.

4. The dissociation of the glycoprotein in the presence of serum proteins that was necessary for quantification was achieved by subjecting the sample to ultrasonication.

5. Assays conducted on a small sample of normal serum produced evidence that normal serum contained amounts (50–180 ng/ml) of a substance that reacted similarly to TH-glycoprotein in the assay procedure and in a series of experiments conducted to confirm the presence of this substance in human serum.

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