U937, THP-1, and J774 cells or human monocytes and macrophages display similar levels of sensitivity to peroxynitrite and exposure to an otherwise non-toxic concentration of the oxidant in the presence of a phospholipase A, inhibitor was invariably associated with the onset of mitochondrial permeability transition (MPT)-dependent toxicity. These events were prevented by exogenous arachidonic acid (AA). In general, the protective concentrations of AA were greater in those cell types releasing more AA. Thus, non-toxic concentrations of peroxynitrite commit cells belonging to the monocyte/macrophage lineage to MPT-dependent toxicity that is however prevented by endogenous AA. (C) 2004 Elsevier Inc. All rights reserved.