In experimental Escherichia coli pyelonephritis, the bacterial multiplication in the kidney parenchyma triggers a burst of neutrophil extravascular migration, as measured by the myeloperoxidase (MPO) activity in the kidney, a marker for tissue neutrophil infiltration. To test the mechanisms of in vivo neutrophil migration, pyelonephritis was surgically induced in rats that were then either complement-depleted with cobra venom factor (CVF), resulting in a profound hypocomplementemia for 72 h after inoculation, or treated with phenylbutazone (PB), a competitive antagonist of bacterial chemotactic formylpeptides. Compared to controls, CVF- and PB-treated animals killed when the neutrophil infiltration started (32 h) had a significantly reduced neutrophil infiltration, as measured by kidney MPO activity. This effect disappeared in animals killed 72 h after surgery, when neutrophil infiltration peaked. These data suggest that redundant chemotactic mechanisms triggered neutrophil migration. Inhibiting one of these mechanisms only transiently delayed neutrophil migration but did not affect the peak infiltration.