<p>In order to prevent the spread of infectious diseases, it is important to prevent the spread of pathogens. Pathogens transmit through frequently hand touched environmental surfaces such as the surfaces of doorknobs and handrails. Introducing antibacterial metallic materials to environmental surfaces is therefore expected to inhibit transmission of pathogens by sterilizing the surfaces. Copper alloys are frequently used because of their high strength, and workability. Among copper alloys, brass is commonly used for environmental surfaces. The brass surface is oxidized by oxygen in the air but few studies have considered the oxides on the brass surface. Since zinc is antibacterial, not only copper oxides but also zinc oxides likely affect the antibacterial properties, and a more effective antibacterial brass surface can presumably be achieved. The object of this study is to propose a treatment method that improves the antibacterial properties of brass surface. Heat–treated brass specimens were immersed in citric acid solution for 24 hours. The specimens were then subjected to X–ray photoelectron spectroscopy analysis, antibacterial tests by the film contact method, and measurements of the amount of ions eluted from specimen surfaces by ICP–MS. It turned out that zinc ions inhibited the antibacterial action of copper ions. The antibacterial mechanism of solid copper seemed to be different from that in the ion solution. An increase in the total ratio of Cu and Cu2O improved the antibacterial properties of brass.</p>