Reactive oxygen and nitrogen species are released by immune-competent cells and contribute to cellular damage. On the other hand, certain pathogens, including Streptococcus pneumoniae, are known to produce hydrogen peroxide (H2O2), while production of nitrogen radicals by bacteria presumably occurs but has been poorly studied. We determined the relative contributions of bacterial versus host-derived oxygen and nitrogen radicals to cellular damage in pneumococcal infection. A special focus was placed on peroxynitrite as a hypothetical common product formed by the reaction of H2O2 and NO. In microglial cultures, reduction of the formation of 3-nitrotyrosine and cellular damage required H2O2-deficient (DeltaspxB or DeltacarB) pneumococci and inhibition of host NO synthesis with aminoguanidine. In infected C57BL/6 mice, neuronal loss and immunopositivity for nitrotyrosine in the dentate gyrus were markedly reduced with DeltaspxB or DeltacarB bacterial mutants and in inducible nitric oxide synthase knockout mice. We conclude that although host and bacteria both produce oxygen and nitrogen radicals, the interplay of prokaryotic H2O2 and eukaryotic NO is a major contributor to cellular damage in pneumococcal meningitis.