We previously demonstrated that 5-amino-7-(3-amino-1-pyrrolidinyl) -1-cyclopropyl-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid (7) has strong in vitro antibacterial activity even against quinolone-resistant bacteria. We examined optimization of the 3-aminopyrrolidine moiety of 7 by introduction of C-alkyl (Me, Et, Pr, di-Me, cyclopropyl) and N-alkyl groups (Me, di-Me). C-Alkylation at the 4-position of the 3-aminopyrrolidine moiety enhanced in vitro and in vivo antibacterial activity. (S)-5-Amino-7-(7-amino-5-azaspiro[2.4]hept-5-yl)-1-cyclopropyl-pyr rolidinyl) -1-cyclopropyl-6-fluoro-1,4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid (15b) showed strong antibacterial activity (in vitro antibacterial activity including quinolone-resistant bacteria is 4 times more potent than that of ciprofloxacin (CPFX) (1); in vivo antibacterial activity is 1.5 to 20 times more potent than that of CPFX (1)) and reduced quinolone toxicity (free from both phototoxicity at a dosage of 30 mg/kg in guinea pigs (i.v.) and convulsion when coadministered with 4-biphenylacetic acid at a dosage of 20 micrograms in rats (i.c.v.)). Their selectivity between DNA topoisomerase II (derived from eukaryotic cells) and DNA gyrase (derived from bacterial cells) was about 3000-fold.