Pyridoxine, like riboflavin, has absorption in the range of near ultraviolet (UVA; 320-400 nm) radiation and is known to decompose after long irradiation with germicidal lamps. Thus, the possibility of UVA-induced pyridoxine photosensitization was studied in cultured normal human, hydroa vacciniforme, and xeroderma pigmentosum fibroblasts. Cytotoxicity caused by the sensitization was measured by post-UVA colony formation. Pyridoxine showed strong cytotoxic effect after UVA radiation and the effect remained for at least 60 min after UVA radiation. Although the cytotoxicity decreased a little when pyridoxine was irradiated under anaerobic conditions, the amount of hydrogen peroxide produced by UVA radiation was hardly cytotoxic and the rate of photodecomposition of pyridoxine was slower under anaerobic conditions than aerobic ones. Thus, the toxicity seemed to depend mostly on the photoproducts of pyridoxine. The UVA-induced pyridoxine cytotoxicity was not due to DNA damage that is to be excision-repaired because group A and C xeroderma pigmentosum fibroblasts were killed as in the case of normal human fibroblasts.