1. Tamm—Horsfall glycoprotein was determined, by radioimmunoassay, in samples of urine from normal individuals under a variety of physiological conditions
2. The amount of glycoprotein excreted in 24 h by our population (39 ± 13 mg, corrected for body surface area) was found not to be influenced by sex, age (19–60 years) or amounts of Ca2+, Mg2+ and Na+ excreted
3. Urine samples collected at 2 h intervals over 24 h from individuals drinking in response to thirst, contained quantities of the glycoprotein which showed high positive correlations with urine volumes, but not with Ca2+, Mg2+ or Na+ excretion
4. The amounts of urine and of the glycoprotein were correlated for individuals in anti- diuresis, induced by restriction of water intake. Relatively small amounts of glycoprotein were excreted by individuals in states of water-induced diuresis
5. The amounts of glycoprotein excreted after exercise were positively correlated with the small volumes of urine voided, but they were uninfluenced by the degree of proteinuria or of hyaline cast formation
6. The half-life for turnover of the glycoprotein in a given individual is highly variable, from a minimum of 3–7 to a maximum of 168 h.