Novel 4-oxoquinoline-3-carboxylic acid compounds of the formula: ##STR1## wherein R.sup.2 is a morpholino group which may be substituted by one to three C.sub.1 -C.sub.6 alkyl groups where each of the alkyl group may be substituted by 1 to 3 substituents selected from the group consisting of --NH.sub.2 and a halogen atom, and R.sup.3 is a C.sub.1 -C.sub.6 alkyl, --CH.sub.2 OH, ##STR2## (C.sub.1 -C.sub.6)alkylthiomethyl and --CH.sub.2 -halogen, or a pharmaceutically acceptable salt thereof, said compounds having excellent antimicrobial activity and hence being useful as an antimicrobial agent, and a pharmaceutical composition containing said compound as an active ingredient.
A series of substituted 4-oxoquinoline-3-carboxylic acids having a methyl group at the 8-position was prepared and tested for their antibacterial activity. 7-(trans-3-Amino-4-methyl-1-pyrrolidinyl)-1-cyclopropyl-1,4-dihydro-6- fluoro-8-methyl-4-oxoquinoline-3-carboxylic acid (21) exhibited highly potent antibacterial activity against both gram-positive and gram-negative bacteria, including Pseudomonas