Little is known about the immunologic aspects of antibody coating, though the test for determining antibody-coated bacteria in urine has been examined for its diagnostic uses by many authors after its inauguration in 1974. In adults with chronic pyelonephritis with and without antibody-coated bacteria in the urine, we tested whether bacterial coating is correlated with the homologous O-antibody titre in the urine. We also determined Ig levels in urine and serum, as well as homologous O-antibody titres in serum. By means of indirect immunofluorescence technique, we were able to show homologous O-antibodies in the urine of all patients with and without antibody-coated bacteria. IgG and IgA levels in urine were mostly raised, as were often the O-antibody titres in the serum. There were no significant differences in the immunologic parameters within the patient groups with or without antibody-coating. The presence of homologous O-antibodies in urine does not therefore necessarily lead to coating of the bacteria.