The local immune response to the lipoprotein of the outer membrane of Escherichia coli O6:K13:H1 was determined in experimental hematogenous pyelonephritis in rabbits. Local antibody was analyzed with the enzyme-linked immunosorbent assay on newly synthesized protein from kidney. Local or intrarenal antibody was detected by day 7 of infection, a few days later than serum antibody. The synthesis of antibody in immunoglobulin G class was present 6 days before the synthesis of immunoglobulin M antibody. Antibody activity to this antigen was also present in urine, and antibody could be eluted from bacteria coated with antibody. Local antibody to the lipoprotein was synthesized in equal quantities by animals infected with a heterologous species of E. coli. This antigen was not as mitogenic for splenic or kidney lymphocytes as was the O antigen. Hence, the lipoprotein of the organisms is a potent immunogen but a weak mitogen locally in pyelonephritis. This antibody response probably does not induce protection against infection, but represents a marker for presence of infection.