Urinary and serum lysozyme activities were determined in patients with acute glomerulonephritis and renal amyloidosis. Serum lysozyme activities were not significantly different than the controls in the patient groups. Although urinary lysozyme activities were similar to those of controls in patients with acute glomerulonephritis, they were significantly increased in renal amyloidosis. Our results suggest that measurement of lysozyme excretion can be used in diagnosis of impaired renal tubular function.