Evaluation of bactericidal effect of different concentrations of ozone when used (a) as a gas, or (b) dissolved in saline. The addition of hydrogen peroxide or 4-hydroxynonenal dissolved in saline was also tested, as well as the effect of human plasma. Staphylococcus aureus, methicillin-resistant Staph. aureus (MRSA), and Pseudomonas aeruginosa, suspended in their culture media were tested. While all bacteria suspended in protein-free saline were killed at high ozone concentrations, they survived when as little as 5% human plasma was present. Hydrogen peroxide was 100-fold less active than ozone and needed to remain in contact with bacteria for at least 60 min. 4-hydroxynonenal (2 mu mol l(-1)) was inhibitory for proliferation of both Staph. aureus and MRSA, but not for Ps. aeruginosa. Ozone and the cascade of its derivative products are potent bactericidal agents, but even small amounts of human plasma, hence of hydro- and liposoluble antioxidants, in bacterial suspensions inhibit oxidation and protect bacteria. Any substantial in vivo cytocidal effect of ozone and its derivatives can be excluded. On the other hand, topical and continuous action of various ozone preparations remains valuable in a variety of skin and mucosal infections.