Ultraviolet-A (UV-A) can damage microbes by generating reactive oxygen species (ROS), singlet oxygen, superoxides, hydrogen peroxide and hydroxyl radicals. These species readily react with lipids, proteins, DNA and other constituents of cells, leading to oxidative deterioration and the eventual death of the microbe. However, the oxidative ability of these reactive species also harms the viability of mammalian cells such as fibroblasts and keratinocytes, as they cause both acute and chronic damage, photo-aging, and photo-carcinogenesis. This study describes a UV-A treatment that does not affect the viability or growth of human neonate dermal fibroblasts, as determined by examining the post-irradiation cell density after the addition of polyphenols as antioxidants. The results demonstrate the possible wide applicability of UV-A sterilization. The potency of polyphenols for attenuating UV-A-induced ROS generation in cells was tested using (+)-catechin hydrate, (-)- epigallocatechin gallate hydrate, morin hydrate, quercetin hydrate and resveratrol. The lowest concentration of polyphenols required to reduce ROS by 50% in cells upon exposure to a dose of 15 J cm(-2) was determined and defined as its IC50. Pre-treatment with morin hydrate at its IC50 allowed cells irradiated with 5.0 J cm(-2) UV-A to recover to the level of the specific growth rate of cells incubated without UV-A irradiation. However, the growth rate of cells exposed to 15 J cm(-2) UV-A irradiation was scarcely influenced by co-incubation with morin hydrate; this dose of UV-A also suppressed cell growth completely in the absence of morin hydrate, although co-incubation resulted in no decrease in cell viability. This study demonstrates the potential of polyphenols for protecting both the viability of cells and their ability to proliferate from damage caused by UV-A-irradiation.