1. Activity of the low molecular weight enzyme lysozyme was measured in the plasma and urine of healthy adults and children and of children with renal disease. 2. No difference was detected between lysozyme excretion (expressed as the lysozyme/creatinine clearance ratio) of healthy adults and neonates, implying that the proximal tubular function of protein reabsorption is mature in the neonate. 3. The lysozyme/creatinine clearance ratio was not elevated in the nephrotic syndrome: thus a heavy load of filtered albumin does not interfere with low molecular weight protein reabsorption. 4. Very high values of lysozyme/creatinine clearance were observed in children with the Fanconi syndrome; there was no overlap with any other group studied. 5. Children with pyuria had a very slight increase in urine lysozyme/creatinine concentration ratio.