ABSTRACTAn attempt was made to establish in vitro and in vivo methods for screening phototoxic compounds. For an in vitro system, light dose‐survival curves ofEscherichia coliwere obtained. 8‐Methoxypsoralen (8‐MOP), bithionol (BT), 3,3′,4′,5‐tetrachlorosalicylanilide (TCSA), and 3,4′,5‐tribromosalicylanilide (TBS) showed photokilling activity againstE. coli, whereas hexachlorophene (HEX), trichlorocarbanilide (TCC), and irgasan CF3were photoinactive. Above a certain critical dose of ultraviolet radiation,E. colitreated with TCSA or TBS exhibited some recovery from phototoxic effects. For an in vivo system, median erythema dose, rather than minimal erythema dose, was determined for guinea pig skin. 8‐MOP, BT and TCSA proved to be phototoxic in both the in vivo and in vitro systems, but in vivo phototoxicity was not shown by HEX, TCC, irgasan CF3, and TBS, thus demonstrating a difference in TBS phototoxicity according t