The appearance of antibody-coated bacteria in urinary sediment has been evaluated in rats with experimental pyelonephritis. At day 7 after induction of pyelonephritis, 9 out of 11 rats demonstrated antibody-coated bacteria in the urinary sediment. The other 2 rats never had a positive urinary sediment. Following removal of the pyelonephritic kidneys, antibody-coated bacteria disappeared in 7 of the 9 previously positive rats. In the 2 rats which continued to show antibody-coated bacteria, the infecting organisms were found in the remaining kidney. The 2 rats which never developed antibody-coated bacteria in urinary sediment had a higher than normal serum antibody titer, and 1 rat with antibody-coated bacteria showed a normal serum antibody titer. It is concluded that although the search for antibody-coated bacteria in urinary sediment is a very useful technique, its negativity does not exclude upper urinary tract involvement; in the case of renal parenchymal infection, immunity is not the same at the systemic and the local site.