Escherichia coli K-12 transformed with pA-CYC184 plasmid DNA was exposed to ozone (O-3) in aqueous solution. The damage to the membrane, protein, plasmid DNA, and cell survival were investigated. Cell viability was unaffected by short-term O-3 exposure (1-5 min) but membrane permeability was compromised as indicated by protein and nucleic acid leakage and lipid oxidation. The intracellular components, protein and DNA, remained intact. With longer durations of O-3 exposure (up to 30 min) cell viability decreased with a more significant increase in lipid oxidation and protein and nucleic acid leakage. The proteins leaking out were further oxidized by O-3. The total intracellular proteins run on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and plasmid DNA run on agarose gel, showed progressive degradation corresponding to the decrease in cell viability. The data indicate that membrane components are the primary targets of O-3 damage with subsequent reactions involving the intracellular components, protein and DNA.