Healthcare associated infections (HAI) pose a major threat to patients admitted to hospitals, and infection rates following orthopaedic arthroplasty surgery are as high as 4%, while the infection rates are even higher after revision surgery. 405 nm High-Intensity Narrow Spectrum (HINS) light has been proven to reduce environmental contamination in hospital isolation rooms, and there is potential to develop this technology for application in orthopaedic surgery. Cultured rat osteoblasts were exposed to 405 nm light to investigate if bactericidal doses of light could be used safely in the presence of mammalian cells. Cell viability was measured by MTT reduction and microscopy techniques, function by alkaline phosphatase activity, and proliferation by the BrdU assay. Exposures of up to a dose of 36 J/cm. 2. had no significant effect on osteoblast cell viability, whilst exposure of a variety of clinically relevant bacteria, to 36 J/cm. 2. resulted in up to 100% kill. Exposure to a higher dose of 54 J/cm. 2. significantly affected the osteoblast cell viability, indicating dose dependency. Work also demonstrated that 405 nm light exposure induces reactive oxygen species (ROS) production in both mammalian and bacterial cells, as shown by fluorescence generated from 6-carboxy-2′,7′-dichlorodihydrofluorescein diacetate dye. The mammalian cells were significantly protected from dying at 54 J/cm. 2. by catalase, which detoxifies H. 2. O. 2. Bacterial cells were significantly protected by sodium pyruvate (H. 2. O. 2. scavenger) and by a combination of free radical scavengers (sodium pyruvate, dimethyl thiourea (·OH scavenger), catalase) at 162 and 324 J/cm. 2. Thus the cytotoxic mechanism of 405 nm light in mammalian cells and bacteria is likely oxidative stress involving predominantly H. 2. O. 2. generation, with other ROS contributing to the damage. Additional work describing the potential for incorporation of this antimicrobial light within operating theatre lighting systems will also be discussed, and this, coupled with the cell viability and cytotoxicity results, suggests that 405 nm light could have great potential for continual patient safe decontamination during orthopaedic replacement surgeries and thereby reduce the incidence of infections