The role of neutrophils (PMN) in acute renal failure (ARF) is controversial. Although the development of acute renal failure (ARF) frequently occurs in situations where there is partial activation of PMN (primed PMN) and mild renal ischemia, the interaction between primed PMN and ischemic organs has not been studied in any biological system. To define the interaction between primed PMN and mild renal ischemia, kidneys were made ischemic for 10 minutes in situ and reperfused by the isolated kidney technique with untreated PMN or PMN primed with low concentrations of lipopolysaccharide (LPS) or phorbol myristate acetate (PMA). We found that primed PMN had no effect on control (non-ischemic) kidneys and that untreated PMN did not cause injury to kidneys previously subjected to mild ischemia. However, addition of primed PMN to mildly ischemic kidneys caused severe injury. To determine the nature of renal injury, ischemic kidneys were reperfused with primed PMN and catalase (CAT) or the elastase inhibitor, Eglin C. In ischemic kidneys reperfused with LPS-primed PMN, Eglin C (but not CAT) was partially protective while in ischemic kidneys reperfused with PMA-primed PMN, CAT (but not Eglin C) was partially protective. Reperfusion with both CAT and Eglin C completely prevented the damaging effects of either LPS- or PMA-primed PMN. In conclusion, addition of primed but not untreated PMN causes ARF in mildly ischemic kidneys by PMN oxidant- and/or protease-mediated mechanisms. This synergism could account for the high frequency of ARF in conditions associated with prerenal azotemia and primed PMN.