T-3262[p-toluenesulfonic acid salt of dl-7-(3-amino-1-pyrrolidinyl)-1-(2,4-difluorophenyl)-6-fluoro-1, 4-dihydro-4-oxo-1, 8-naphthyridine-3-carboxylic acid monohydrate] is a new pyridone carboxylic acid with a broad spectrum of antibacterial activity against Gram-positive and Gram-negative bacteria. The activity of T-3262 against most Enterobacteriaceae was comparable with that of ciprofloxacin except Proteus spp. and Providencia rettgeri and exceeded that of ofloxacin and norfloxacin. The activity of T-3262 against Pseudomonas aeruginosa was comparable with that of ciprofloxacin, and T-3262 was more active than the other new quinolones against Acinetobacter calcoaceticus, Branhamella catarrhalis and Haemophilus influenzae, and also against staphylococci, streptococci, and Bacteroides fragilis. The protective effects of a single oral dose of T-3262 on systemic infection in mice were greater than norfloxacin. T-3262 was as effective as ofloxacin and ciprofloxacin against systemic infections in an animal model with Escherichia coli and Klebsiella pneumoniae, and more active against Ps. aeruginosa infections. T-3262 showed excellent activity against staphylococcal and streptococcal infections.