Two optically active isomers of 1‐ethyl‐6,8‐difluoro‐1,4‐dihydro‐7‐(3‐hydroxypyrrolidin‐1‐yl)‐4‐oxoquinoline‐3‐carboxylic acid (10) were prepared. One of the isomer, 7‐[(3S)‐hydroxypyrrolidin‐1‐yl] derivative 8, was about 4 times more potent in vitro than the other, 7‐[(3R)‐hydroxypyrrolidin‐1‐yl] derivative 4, and approximately two times more active than the racemate, 7‐[(3RS)‐hydroxypyrrolidine‐1‐yl] derivative 10. Optical active 8 was the most active in in vivo, followed by 10, and 4 was the least active compound. But, they were more potent than CI‐934 12 and norfloxacin. From the results, (3S)‐hydroxypyrrolidinyl group was found to be one of the beneficial group for PCA‐anti‐bacterial agent.

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