The phototoxic potentials of quinolone antibacterial agents were investigated in Balb/c strain mice. The mice were orally administered nalidixic acid (NA), enoxacin (ENX), ofloxacin (OFLX), ciprofloxacin (CPFX), lomefloxacin (LMFX) and DR-3355 (s-isomer of OFLX), and immediately exposed to ultraviolet-A (UVA) for 4 h (21.6 joules/cm2). The ears were examined for overt damage, as a major phototoxic parameter, 0, 24 and 48 h after irradiation ended. At doses of 200 mg/kg, LMFX, NA and ENX caused marked cutaneous phototoxic reactions on the ears, whereas CPFX, OFLX and DR-3355 caused none. At 800 mg/kg, however, CPFX, OFLX and DR-3355 also caused phototoxic reactions on the ears. These phototoxic changes were characterized grossly by erythema, and histopathologically by edema and infiltration of inflammatory cells, especially neutrophils, into the connective tissue surrounding the cartilage. The 50% erythema-inducing doses of LMFX, ENX, NA, OFLX, DR-3355 and CPFX were calculated at 19, 102, 143, 553, 619 and 741 mg/kg, respectively. Thus, the phototoxic potencies of the quinolones tested were: LMFX greater than ENX, NA greater than OFLX, DR-3355, CPFX.