Urine samples of 17 patients with nephropathy were tested with precipitating anti-thermostable-kidney antigen sera (anti-kidney BE antigen), which had been made kidney specific by absorption with diverse tissues. Kidney BE antigen was demonstrated in the concentrated urine of a patient (case I.T.) with membranous nephropathy, in whom membranous nephropathy appeared to be associated with exposure to chromium. Kidney BE antigen was, however, not demonstrated in the concentrated urine from the other 16 patients with nephropathy (chronic glomerulonephritis, 7; acute glomerulonephritis, 3; lupus nephritis, 3; membranous nephropathy, 2; diabetic nephropathy, 1). Kidney BE antigen was localized in the tubular cells but not in the glomeruli, by indirect immunofluorescent staining. Kidney BE antigen was not demonstrated in the glomeruli in case I.T., even after acid elution of the sections. The possible role of kidney BE antigen in the pathogenesis of membranous nephropathy is discussed.