Serum and urinary lysozyme values were assayed in 51 patients with chronic glomerulonephritis and 10 undergoing regular hemodialysis. Renal threshold for lysozyme was in the range from 15 to 20 μ/ml, but no correlation was obtained between serum and urinary lysozyme concentrations. Significant correlations were found between serum lysozyme activities and commonly employed parameters of renal dysfunctions, such as blood urea nitrogen level (r=0.77), serum creatinine level (r=0.86), and creatinine clearance (r=-0.80). The majority of patients with markedly diminished glomerular filtration rates smaller than 20 ml/min exhibited urinary excretion of more than 3 mg of lysozyme per day and elevated serum lysozyme, while in patients with mildly to moderately impaired renal function, serum and urine lysozyme concentrations were within normal limits. Therefore, positive lysozymuria and increased serum lysozyme activity may be taken as evidence of severe renal damage, whereas the lysozyme determination may not serve as sensitive indicators in mildly to moderately injured renal diseases.