The toxicity of peroxynitrite toward Escherichia coli (expressed as LD₅₀, the concentration required to kill 50% of the bacteria) was found to be independent of bacterial cell densities over a wide experimental range, spanning 10⁶−10¹⁰ colony-forming units/mL; the magnitude of LD₅₀ was also pH-independent over the range pH 5.9−8.3. This highly unusual behavior can be quantitatively reproduced by a dynamical model in which (i) ONO₂H is identified as the toxic form of the oxidant and (ii) the bulk of the added peroxynitrite decays to nitrate ion under these conditions. From the model, one estimates that 10⁶−10⁷ ONO₂H molecules are required to kill a bacterium, indicating a very high intrinsic toxicity (cf. HOCl, for which LD₅₀ = 10⁷−10⁸ molecules/cell of E. coli). Nearly complete protection was observed when bicarbonate ion was added to the buffer, even when concentrations of peroxynitrite exceeded 50 times the LD₅₀ measured in the absence of bicarbonate. Consistent with previous reports, combinations of H₂O₂ and ^•NO and, in weakly acidic media, H₂O₂ and NO₂^- were found to exhibit enhanced toxicities relative to the individual reactants. Protection by bicarbonate was utilized to assess the potential role of intermediary formation of ONO₂H in bacterial killing in these systems. Approximately 25% protection by bicarbonate was observed for media containing H₂O₂ and NO₂^-, consistent with a minor contribution to killing by ONO₂H under the experimental conditions. No protection was observed for media containing H₂O₂ and ^•NO in both anaerobic and aerobic environments, excluding extracellularly generated ONO₂H as a participant in these bactericidal reactions.