A group of 37 hemodialyzed patients with chronic renal failure were investigated within a period of 6 months to 9 years. Of these patients five presented quantitatively significant bacteriuria noncorrelated with the causal nephropathy. Of the 9 patients with urologic past history two presented urinary infection. Urinary bacterial immunofluorescence was constantly positive in all the cases in which bacteriuria was significant and, at least in one determination when the germ count was below 10,000/ml. This finding was also supported by the increase of gammaglobulinemia and of the circulating immune complexes in all the cases investigated as well as of the B lymphocytes in one third of cases. Fifty percent of the patients presented T lymphocytopenia with excess of suppressor lymphocytes and deficit of the helper ones. It can, therefore be assumed that in patients with chronic renal failure nondialyzable antigens persist in the renal parenchyma where they stimulate anticorpogenesis but at a low level owing to some immune disturbances present in such patients.