A highly sensitive enzyme immunoassay system for the determination of cuprozinc-superoxide dismutase (Cu, Zn-SOD) in serum and urine by using β-galactosidase as a labeling enzyme is reported. This assay had a greater sensitivity than that of previously reported radioimmunoassay methods and could measure from 0.05 to 10.0 ng of Cu, Zn-SOD with good reproducibility. Coefficients of variation for this enzyme immunoassay were 2.8-8.3% within a day and 5.6-16.1% between days. The average recoveries were 96-103% from sera and 96-105% from urines, respectively. Using this enzyme immunoassay, serum Cu, Zn-SOD concentrations increased significantly in patients with renal diseases and were slightly elevated in patients with liver diseases. Urinary Cu, Zn-SOD appeared to be the highest in all renal diseases. By immunofluorescent staining, Cu, Zn-SOD was located in the thickened portions of the glomerular basement membrane and proliferating mesangial cells of the kidney tissue from a patient with membranous glomerulonephritis.