A recently developed, simple and sensitive radioimmunoassay has been used to examine 24 h excretion and plasma levels of Tamm-Horsfall glycoprotein (THG) in normal subjects, stone formers and patients with stable chronic renal disease. In normal subjects THG excretion ranged from 22 to 66 mg/24 h, with no sex difference and no correlation with creatinine clearance or body surface area. There was no correlation between 24 h THG excretion and urine volume, pH or osmolality, excretion of Na+, K+ or Ca2+ or free-water clearance. There was a small significant correlation between plasma THG concentration and urinary THG excretion. A good correlation was obtained between the THG/creatinine ratio in 24 h and random samples. This made possible the use of random samples to establish a reference range for THG excretion of 0.15-0.50 micrograms/ml of creatinine clearance which did not depend on sex or age. The excretion rate of THG in stone formers was generally within the reference range. It was not significantly different in those who were hypercalciuric or in those taking thiazides. In patients with chronic renal disease there was a good correlation between 24 h THG excretion, plasma THG concentration and creatinine clearance. The range of excretion of THG per ml of creatinine clearance was greater than in normal subjects, independent of the type of renal disease and unrelated to proteinuria. In patients with glomerulonephritis the excretion of THG per ml of creatinine clearance was significantly higher in those with well-preserved tubules compared with those with tubular atrophy.