Host defense against bacterial pathogens in higher organisms is mediated in part by the generation of reactive oxygen species (ROS) by PMN. In this study, we determined the following effects of exposure of constant concentrations of H2O2 on E. coli in a culture continuously monitored for H2O2 concentration, numbers, and viabilities of cells: (1) E. coli growth rates monitored for 1 h were profoundly affected by concentrations of H2O2, between 25-50 microM. (2) Complete bacteriostasis was observed at 100 microM. (3) Significant cell killing was not observed until the concentration of H2O2 was greater than 500 microM. (4) Bacteriostatic (25-50 microM) concentrations of H2O2 appeared not to be toxic to human skin fibroblasts for a 2-h exposure. (4) Bacteriostasis by H2O2 could not be explained by metabolic inhibition, because intracellular ATP levels were not compromised at bacteriostatic doses of H2O2. (5) Measurements of H2O2 concentrations in subcutaneous abscess fluid infected with both E. coli and S. aureus indicated prevailing concentrations of the oxidant consistent with a proposed role of H2O2 in host defense.