Skin phototoxicity (photoirritation) is defined as an acute toxic response elicited by topically or systemically administered photoreactive chemicals after the exposure of the skin to environmental light. The in vitro reconstructed human epidermis phototoxicity test (RhE PT) is used to identify the phototoxic potential of a test chemical after topical application in reconstructed human epidermis (RhE) tissues in the presence and absence of simulated sunlight. Phototoxicity potential is evaluated by the relative reduction in viability of cells exposed to the test chemical in the presence as compared to the absence of simulated sunlight. Chemicals identified as positive in this test may be phototoxic in vivo following topical application to the skin, eyes, and other external light-exposed epithelia.