DR-3355 [S-(-)-ofloxacin], an optically active isomer of ofloxacin, showed a broad spectrum of antibacterial activity against both Gram-positive and Gram-negative bacteria and was generally twice as potent as ofloxacin and considerably more active than DR-3354 [R-(+)-ofloxacin]. The activity of DR-3355 was largely unaffected by the type of culture medium, inoculum size and addition of human serum, but decreased under acidic condition at pH 6.0. The protective effect of orally administered DR-3355 in experimental infections in mice with various bacterial pathogens was superior to those of ofloxacin and ciprofloxacin.