The urinary excretion of adenosine-deaminase-binding protein, a constituent of the brush border of proximal renal tubule cells, has been investigated in 39 patients with disorders associated with malfunction of the renal tubules, and its excretion has been compared with that of two low molecular mass plasma proteins and an enzyme derived from renal tubular cells. None of the 36 patients with disorders associated with chronic renal tubular malfunction were found to be excreting significantly increased quantities of adenosine-deaminase-binding protein but 30 had increased excretion of retinol-binding protein, alpha 1-microglobulin, or N-acetyl-beta-D-glucosaminidase. Measurement of urinary adenosine-deaminase-binding protein may be useful in the assessment of acute renal tubular injuries but it is not of value in the detection of chronic renal tubular disorders.