A number of 7-[(3-methylthio or methylthiomethyl)-3-pyrrolinyl] quinolone-3-carboxylic acids were synthesised by condensation of 7-fluoro substituted quinolone-3-carboxylic acid with 3-methylthio-3-pyrroline or 3-methylthiomethyl-3-pyrroline. The in vitro antimicrobial activity of them were tested against twenty species of Gram-positive or Gram-negative microorganisms. It showed remarkable antibacterial activity, particularly against Gram-positive microoganisms. Among those 1-cyclopropyl-6,8-difluoro-7-[(3-methylthiomethyl) -3-pyrrolinyl]-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid(12a) and 1-cycl opropyl-6-fluoro-8-chlore-7-[(3-methylthiomethyl)-3-pyrrolinly]-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid(12b) showed the most potent in vitro antibacterial activity.